When you buy something from the mobile iTunes store, it downloads to the device and places into the "Purchased" playlist by default (and if it doesn't exist, it makes one)
Now I just bought a song, and from the Downloads section of the iTunes store pressed on the "Purchased" shortcut button (the one in black). And it opened up a list of all songs on my iPod (so basically opened up the "Songs" section in Music).
So tried accessing the "Purchased" playlist direct from the Playlist section of the Music program. You know what? It opened "Songs".
A restart didn't help... so sounds like a bug to me!
Anyone buy music with your iPhone / iPod touch 2.1 lately? It definitely did not happen before, I can assure you that!