Originally Posted by EricF
Depending on what signals are being run through the PA board, you might be able to take a signal from the tape-outs and get a decent mix. If it's a small club situation, however, it's likely that only the vox will be in the PA, and you won't get any of the instruments in your mix (except what bleeds through the vox mics) unless you mic them seperately, which adds dramatically to your cost, complexity, and setup/tear-down time.
I have used a portable recorder (Zoom H4n, in my case) to record band rehearsals and live shows. The built-in mics work very well, but I've also had good success with a stereo ribbon mic placed to get a good balance from the instruments and the PA. From there, you can drop the .wav file into your editing software (Cubase, ProTools, etc.) and make any edits or EQ/processing tweaks you want.
If you're going from the tape-outs on the mixer, then why even bother with a laptop, which is cumbersome, fragile and takes up space and needs power? I'd just go directly into the line-in of a digital recorder. If you wanted to keep tracks separate for later mix-down, then you'd need a USB interface.
I use a Yamaha Pocketrak for live recording and it, the Zoom and related digital recorders work well. It might look cool to have a computer up on stage with you, but it's just a big hassle.