Starting life as a research project called MySong at Microsoft, Sumit and Dan analyzed melody to create backing tracks to a recorded mic input.
Now called Songsmith, they've teamed up with Garritan for the sounds and Band-in-a-Box for the styles and playback engine. The software analyzes a recorded input then puts chords, bass and a beat to it. Once recorded and analyzed, you have control over the style of the backing track by moving 'happy' and 'jazzy' sliders to change the chords, you can also ask it to suggest other chords that would work with the melody.
The results can be exported WAV or WMA the backing track, the vocal and also the MIDI data for importing into other DAW software. It's actually very cool, and you know it's only $30!!!
Dan and Sumit don't have plans to port this to any other platforms than Windows for now, but if it blows up really big then you never know.