I've been using Melodyne Editor to convert monophonic and polyphonic RTs to midi with some great results. It's not perfect and can't handle the kitchen sink but depending on the source material, it can be quite worthwhile (I also used it to turn my voice into a convincing flute -- way cool!)

Where it struggles is with things like pitch bends, heavy reverb (I get audio as dry as possible before melodyning-to-midi) and non-pitched noises like aggressive thwacking in strummed guitar parts, for example. It can also help a lot if, before converting to midi, you "flatten" the audio by reducing / eliminating vibrato and pitch drift using Melodyne's Pitch Modulation and Pitch Drift tools.

Not cheap software, but very very impressive, in this regard as well as other.

http://www.youtube.com/watch?v=3tDj_VanH3g