Being a zealot for both TiVo and Apple can be tough at times. We can't watch TiVoToGo files yet, even though the CEO is a switcher. We have to listen to people constantly telling us about how the companies are about to die. At least now people who encode their music in Apple's AAC format have a way to play their music on their TiVos through TiVo Desktop 1.9.
According to Dennis Wilkinson on the TiVo Community forums, 1.9 includes a program called "SoundConvert" that will run AACs through LAME, if LAME is installed in /usr/local/bin/. All you need to do is install LAME and restart the TiVo Desktop. macosxhints has a guide to installing LAME, or you can get it from Vas the Man.
Unfortunately, you can't play songs you bought from the iTunes Music Store because they have DRM. If you thought you should actually be able to listen to music you bought on your TiVo, you'd need to strip out the DRM with something like jHymn.