ok... Yes, you can, but no, it's not really worth it...
You can get plenty of converters from all over the web, try
WebAttack.
But, the resulting *.mid file is ussually shocking quality and sounds nothing like the original...
You must remember, *.mid files are simply a set of instructions, nothing more, no sound data is contained wihtin them at all...
These sets of instructions simply comunicate wiht the native MIDI database which is on the computer, and tells it which sounds to play out of a set few...
Therefore, getting perfect bass sounds, or really good guitar riffs in *.mid files after being converted from mp3 is virtually impossible...
BUT! don't hold me to my words, because i know that there are other people out there on these forums with MUCH more knoledge on sounds and media than me, so i could be wrong.. let's wait and see...
Hope I Helped...

Team EOD :: Programmer/Logical Engineer/All-Round Nice Guy