I'd quite like it, but it would cause a few issues.. How would you start / stop it? Awkward button combinations?
It'd probably cause the game to lag a little, if the PS3 is processing another program on top of the game.. Using the XMB would be a nightmare.
& if anything like that were to come out, I'll guarantee you it'd be on a PS3 firmware update

(3.00 on September first (does NOT contain that))