Ah, I should have been clearer. MIDI to sheet music conversion is trivial. What I want to do is take real audio as input and convert that, which is according to my readings really hard because MIDI and audio formats (FLAC/WAV/MP3 etc) are very different.
There are some tools that I could find but so far none has come up with good conversion from FLAC or MP3.
I'm mainly interested in classical music and that kind of complicates things. As the number of voices in your audio increase, it gets that much harder to export it to MIDI. I've been thinking about emphasising the melody line and exporting that to MIDI but so far nothing :P
The research field trying to tackle this is called "music information retrieval", that may help with your search.. I think they have made some headway but it involves quite a few challenges - auditory scene analysis, source separation, pitch labelling etc.
midisheetmusic.sourceforge.net
Hell, just upload your MIDI to hamie.net and they'll do it for you (albeit horrendously).