FOUND IT (edit
well kind of) ;_; <-- tears of happiness (edit
actually tears of sadness it turns out)
Go into your Winamp.ini (.../AppData/Roaming/Winamp/Winamp.ini) and add to the [ml_ipod] section 'showAllEncoders=1'. I was reading through the source code, and it looks like ml_ipod is looking for a codec starting with 'M4A' or 'MP3' (case-sensitive), so I'm guessing Winamp renamed the AAC codecs. Just be sure to not do something stupid like use FLAC
This was the only reason I jumped ship to pmp_ipod.dll, but that just fucked me over royally so I needed to be able to come back. I'll file a bug report and this should be a pretty painless fix.
The songs are not being transcoded properly for some reason. I used the transcoder with AAC with pmp_ipod.dll and it worked fine, but now not so much.