I have a few hundred tracks purchased from the iTunes store that are managed by MC10 (.32 is current). The program shows many but not all of the tag info.
What shows is: track#, name, album, composer, bitrate (which, BTW, indicates 125k instead of 128k.
What does not show is: genre, date (year), last played, number plays, comments, ), and 'my rating'.
The tracks under test had empty fields filled by me in the iTunes manager (for Windows) and auto-filled fields were filled by playing tracks in iTunes mgr (to fill 'last played' and 'number plays').
With HP announcing the remarketing of the iPod and support for the iTunes Store, 'AAC' is here to stay in a big way. With 'AAC' support in MC10 via QuickTime codec and encoding available via 'iTunes Manager' (or 'Nero 6'), this is starting to become a workable choice for encoding music.
Ultimately, I would image that J River could simply sell an AAC codec plug-in to interested users for whatever Apple charges plus a markup. Let's say it was $40, I believe enough interested users of MC would buy it.
I am sure 'AAC' file support is still a work in progress but thought this might be helpful feedback.