Apple has released a new app called Music Memos that lets you record basic songs on your iPhone, edit them a little, and then if you like, port them to GarageBand for heavy duty changes. It seems to be intended for sketching out song ideas -- particularly on acoustic guitar and piano -- when you're out and about, although the tools included and uncompressed, high-quality audio let you put together an impressive song just with the app itself, especially if you elect to connect an external mic.
Once you record the song, you can add in a virtual drummer and bassist which play along intelligently, speeding up or slowing down as appropriate. Hit the source for demos of this in action -- the result is surprisingly professional.
Tags and ratings help you organize your body of work while sharing controls let you send it to band members, friends, your iTunes library, or the web.
Memos is a free app. Click here to download it.