Mac users have long been able to do this by installing the LAME encoder on the Mac. The Mac TiVo Desktop will use LAME to transcode AAC to MP3 for playback on the TiVo. But Windows users have been out in the cold.
Well, long time TiVo user and software developer
Dan203 just released
a plugin for TiVo Desktop on Windows that allows you to play unencrypted AAC files via the TiVo. It transcodes them to MP3 on the fly to stream to the TiVo's standard music playback system.
Note: This cannot play FairPlay encrypted AAC files purchased from iTunes, only unencrypted AAC that you've ripped yourself. (Or iTunes files you've stripped the DRM off of with JHymn, or the like.)
And if you didn't already know, another TiVo user,
pzand, previously released
a plugin for playing back WMA files. I'm going to have to try out the AAC plugin, I ripped all my CDs into AAC a while back, and between that and iTunes I have over 12,000 tracks. (All legal.)