There are a few tools that claim to do this:
http://www.kichiki.com/WAON/waon.html
http://www.audioto.com/
I tried this with irritating, unpredictable, and useless results for precisely the same purpose. YMMV.
You are going to laugh at this, but here was my solution;
Yamaha Ez-Eg
Yes, it is a toy, and not inexpensive one at that (the "acoustic" version, the Ez-Ag, is cheaper). But, with a little manipulation, it works as a decent midi controller. It has decent midi implementation and comes with a MIDI-to-USB adaptor. Unlike a roland guitar synth pickup, there is no pitch-to-midi conversion. Six "buttons" at each fret position act as note-on triggers. Six "string" triggers at the "bridge" trigger the notes. Thus, there isn't the lag or tracking errors often associated with midi pickups. It isn't perfect. I bought mine because I sometimes have to program midi guitar (yes, I actually HAVE to program midi guitar), and because it's a slightly more familiar interface than a keyboard to me. Sometimes I have to do a little cleanup after playing it in, but it's worth it for me. It can be kind of fun playing pad and synth sounds with. It's one of those things that looks kinda ridiculous but kinda cool at the same time (light-up frets!

).
In order for it to be really effective, you need this utility to manipulate the incoming data:
http://www.tencrazy.com/gadgets/ezeg/
I actually use another one called MIDIX.hta but the site seems to be down. You can get a bunch of info by reading through the HUGE thread at KVR:
http://www.kvraudio.com/forum/viewtopic.php?t=41787