I have both MP3 and AAC files, all tagged with standard ID3 tags (done in iTunes).
Are you sure iTunes is using ID3 tagging for the AAC files? It sounds like its using QT tagging for the AAC files.
The default music application on the PhotoBridge only supports ID3 tags as you know, so assuming the AAC files are tagged with QT tags then you'll only get the file name.
If you want to see the actual tags for the files you'll have to use a third party application like SlimRoku (using SlimServer as a server) or Sonica (which can just use shares like the default music application).