I frequently fire up utorrent while using 2 miners in GUIminer. I haven't seen a performance issue at all.
I also use news groups and when downloading from an nzb I have to throttle it a bit but that's a whole different monster.
It's the exact opposite for me. I run two miners as well but I have to turn off the first one while I'm using this PC for anything other than just mining, or else it will freeze up. Granted, both of my GPUs are overclocked 30%, so that may be the reason for the freeze.