Also see the Apple iPad page, which covers iOS 5 and briefly compares iPad and iPhone. 

A major difference is that the iPhone does not support importing photos from directly from cameras or SD cards using Apple's camera connection kit. I have not tried WIFI memory cards, cameras, or devices such as Airstash, but transfer of camera images to the iPhone should be possible with those.

This page will mention apps for the iPhone for cataloging or exporting images from the iPhone to email, Flickr, etc. The emphasis is on metadata preservation. The list is not at all definitive.

Sampler of iPhone apps for field notes

Taking notes in the field can help with cataloging later.

• An efficient way is to use apps that incorporate the iPhone's camera and allow you to add metadata or commentary to pictures you take with app. 
• The iPhone 4S (and iPad 3rd generation) allow the dictation of text within apps as an alternative to typing. Dictation requires and internet connection and low ambient noise.

Examples of methods are: 

Photogene2

• Add IPTC IIM metadata to pictures taken in the app. 

Evernote

• Attaches written and audio notes to a picture.
• Audio notes are especially helpful if dictation is not feasible for writing text.
• Maps location and provides approximate address.
• The free version of Evernote is a cloud service. A premium version for offline use is available for a fee. 

neu.Notes

• Places photo on a page in a notebook
• Write freehand notes, type or dictate, add map.

Adobe Ideas

• Take a picture and write freehand on the photo. 

FotoNote

• OCR app that converts text in photos to editable text.
• Useful for signs, labels, etc., for anyone who would rather copy and paste text than transcribe it. 

There are also apps for metadata, notes, or journals that can import photos from the Camera Roll and other albums.

iPhone apps for reading or writing metadata

Flickd

• Exports to Flickr without loss of metadata.
• Can send a tweet and a link to a photo via Twitter.  
• The link has to be copied from another source, such as Safari, and pasted.
  
• The app includes a camera function
• It opens to the camera
• That means it is set up to take a picture and get if off to Flickr or Twitter right away
• The opening page also includes an icon to access the iPhone photo library, so you can export pictures from the library instead of the camera.
  
• Includes Flickr fields for Title, Description, and Keywords
• These are Flickr fields, not the comparable IPTC fields.
• Exporting from iPhone import folders (albums):
• These are the folders for images imported via the camera connection adapters.
• They are the only folders that are exempt from the iOS metadata restrictions.
• The images have all of their technical metadata in tact.
• There are no images with IPTC metadata, because it is not possible to save images to the import folders after entering metadata.
• All of the technical metadata is exported intact.
• Exporting from other albums
• The Camera Roll and all other folders observe the Apple metadata restrictions.
• For IPTC, the folders are restricted to IIM fields or fields that map to IIM.
• Flickd exports all metadata that the image contains.
• If the image has been processed in another app for adjustments or cataloging, the image contains the metadata that the app exports. 
• For the Flickr display, the Flickr fields have priority over IPTC fields.
• Flickd entries do not conflict with previous IPTC  entries.
• The title, description, and keywords entered in Flickd display with the image.
• IPTC entries can be seen on the Exif Info page.
• You can catalog and export one image at a time. Flickd is not for bulk processing.
• The app stores keywords for future use. It does not have other templates or a memory for other fields.
• If you shoot raw files, Raw+JPG works best for Flickd.
• Flickd exports a JPG image.
• For Raw+JPG, it exports the JPG
• For Raw, it exports a thumbnail
• In both cases the camera metadata exports in full, while the image metadata is for the JPG.

PhotoNotes

• Reads, writes and edits some descriptive and technical metatada.
• Descriptive metadata:
• EXIF comment
• TIFF description, document name, and copyright.
• IPTC keywords
• These fields map to various schemas in Exif Viewer Firefox plugin and Adobe File Info.
• Except for TIFF document name, which I couldn't find in Exif Viewer or the File Info advanced panel.
• The that fields that show up in other metadata readers are comment, description, copyright, and keywords.
• All fields show up, in the same schemas as in PhotoNotes, in a metadata reading app for the iPhone called "Metadata"
• Technical metadata can be edited:
• make, model, software, date, latitude, longitude, ref N and W, and altitude.
• Entering copyright and description overwrites comparable IPTC fields, even though the app does not read existing IPTC entries. It only reads IPTC keywords.
• The app does not harm existing technical metadata.

Photo Metadata Reader

• App that reads IPTC IIM and technical metadata.

• Reads all metadata written in PhotoNotes and categorizes them according to the same schemas.
  

   

Photogene2 for iPhone

• Photo adjustment app similar to the iPad version.
• It reads and writes IPTC IIM and reads EXIF and GPS.
• GPS places a pin on a map instead of showing coordinates.

Filterstorm for iPhone

• Photo adjustment app.

• Writes IPTC IIM.
• Filterstorm Pro for the iPad writes IPTC Core instead of IIM.

GPS Data Tracking 

Apps that use the GPS capabilities of the 3G iPad or iPhone to track locations. That is, the apps turn the mobile device into a data logger that exports GSX files that can be use to embed GPS metadata.

Note: iPads with 3G are the only iPads with GPS. There does not need to be a 3G connection for GPS to work.

MyTracks

• The GSX files can be used on any platform, but this app as a whole is intended to work with iPhoto, Aperture, and MyTracks, a Mac mapping program.  

Motion-X GPS

• Creates tracks and exports them as GSX files. 

Garmins and Tom Toms also create tracking files that can be used if the device attaches to a computer. They aren't iPhones, obviously, but while on the subject of embedding GPS data, are worth a mention.