Look for something that uses windows directshow filters (like windows media player or BSPlayer) or is based on winamp, as they all have AAC decoders available.
Chris: I thought MP4 (and hence AAC) was an ISO standard (as its part of the mpeg 4 spec?) and as such is not protected? Presumably this is why open source decoders/encoders are available? I could be way wrong here....
I think the lack of aac support is due to the dominance of mp3 and the fact that aac is "relatively" new. Apples Ipod has done more to bring AAC to the fore than anything else, which is probably a good thing.
If everything used 256kbps to encode as standard I'm sure we would not have so many audio codecs.....