I've heard of programs like OMB and Reason that permit live aarranger-type playing. Is that true? How do they work? I'm pretty PC savy, but getting into that has me wondering.

I also asked the same question and got most of the answers by actually downloading the demo version and tried it out. I was able to record a song with the rythms into a midifile in like 10 minutes or so. Since I did not have a cable to hookup my keybd, I just used the pc keys and the terrible internal pc sound card, kind of awkward but it illustrates the concept.

Here is the link where you can download the software
http://www.home.zonnet.nl/JosMaas/onemanband/index.htm

My question to Jos the creater of OMB is, does the software allow using voices/sounds from the popular softsynths or strictly the XG sound set ?