Where is my data stored?

All information entered using iTag is written to the IPTC and XMP headers of each file.
Below is an indication of where iTag stores the captions for each file type and what applications can also see this information:

File Type IPTC XMP
JPEG
RAW, TIFF, PNG  
AVI, MP4  
MP3, WAV  

Metadata Engine

All files supported by iTag are read and written using the Adobe XMP Toolkit since build 374. Older versions of iTag used Microsoft's Windows Imaging Component however it proved to have many problems and Microsoft have been slow to release fixes. Another major advantage of the Adobe XMP Toolkit is that it does not offset the EXIF Makernotes as WIC does.

AVI Compatibility

I don't know of any other application currently capable of performing AVI tagging but if you want to verify the XMP is in the file, open up a (small!) avi in your favorite text editor and search for "xmp".

RAW Compatibility

Depending on the particular format, some Microsoft products such as Pro Photo Tools and Windows Live Photo Gallery may be able to read the data too but their RAW support is lagging somewhat. Adobe Photoshop should have no issues finding the metadata.

JPEG Compatibility

Below is a table that lists the known applications that all work with JPEG IPTC and XMP headers

Application Comments XMP Storage Location
Title Description Tags
Desktop Applications
iTag 100% compatible with itself! /xmp/dc:title /xmp/exif:UserComment
/xmp/dc:description
/xmp/dc:subject
Adobe Photoshop CS3 Fully compatible /xmp/dc:title /xmp/dc:description /xmp/dc:subject
Adobe Lightroom Fully compatible /xmp/dc:title /xmp/dc:description /xmp/dc:subject
GeoSetter Fully compatible /xmp/dc:title /xmp/dc:description /xmp/dc:subject
GraphicConverter (Mac) Fully compatible /xmp/dc:title /xmp/dc:description /xmp/dc:subject
Windows Live Photo Gallery Fully compatible /xmp/dc:title /xmp/exif:UserComment /xmp/dc:subject
Microsoft Pro Photo Tools
(formerly Photo Info)
Fully compatible /xmp/dc:title /xmp/exif:UserComment /xmp/dc:subject
Vista Flickr Uploader Fully compatible /xmp/dc:title /xmp/exif:UserComment /xmp/dc:subject
Application Comments IPTC Storage Location
Title Description Tags
Desktop Applications
iTag 100% compatible with itself! IPTC.Headline &
IPTC.ObjectName
IPTC.Caption
JPEG.Comment (read only)
IPTC.Keywords  
Adobe Photoshop CS3 Fully compatible      
Adobe Lightroom Fully compatible      
BreezeBrowser Pro 1.7.2 Fully compatible      
Copernic Desktop Search 2 Fully compatible      
Exifer Fully compatible      
ExifTool Fully compatible      
Exiv2 Fully compatible      
GeoSetter Fully compatible      
GraphicConverter (Mac) Fully compatible      
IDimager Fully compatible      
IrfanView Fully compatible      
Mapivi Fully compatible      
Opanda IExif Fully compatible
(Editing requires PowerExif which isn't free)
     
Photo Mechanic Fully compatible      
Pixvue [Discontinued] Fully compatible IPTC.ObjectName IPTC.Caption IPTC.Keywords
XnView Fully compatible      
Photo Album Viewer/Generator
Adobe Photoshop Album 3.2 Not compatible.
Metadata appears to be stored in proprietary database
     
JAlbum Fully compatible when using Chameleon skin      
J.River Media Center Title is ignored.
Caption & Tags are automatically read in.
- IPTC.Caption IPTC.Keywords
Picasa Title is ignored.
Caption & Tags are automatically read in.
- IPTC.Caption IPTC.Keywords
Cooliris Compatible when gallery is generated by iTag     -
Porta Fully compatible      
Web based Applications
Flickr Fully compatible      
Fotki Fully compatible      
Panoramio Geocode compatible - - -
Picasa Web Albums Title is ignored.
Caption & Tags are automatically read in.
- IPTC.Caption IPTC.Keywords
SmugMug Title is ignored.
Caption & Tags are automatically read in.
- IPTC.Caption IPTC.Keywords
Zooomr Fully compatible      
Photo Uploaders
Flickr Uploadr Fully compatible      
JUploadr Title is ignored.
Caption & Tags are automatically read in.
- IPTC.Caption IPTC.Keywords
Vista Flickr Uploader Fully compatible IPTC.ObjectName IPTC.Caption IPTC.Keywords

Besides Title, Description and Tags - iTag allows the editing of several other fields. Their respective storage locations are listed below:

Other iTag fields
Author /xmp/dc:creator
IPTC.Byline
Copyright /xmp/dc:rights
IPTC.Copyright
Rating /xmp/xmp:rating
/xmp/MicrosoftPhoto:Rating
GPS Latitude EXIF.GPSLatitude, EXIF.GPSLatitudeRef
/xmp/exif:GPSLatitude
GPS Longitude EXIF.GPSLongitude, EXIF.GPSLongitudeRef
/xmp/exif:GPSLongitude

Let me know of any errata or if you use a different application that works with IPTC or XMP.