no way, sell the mac pro if you're really intent on getting rid of it. if you don't need the mobility all you're doing is losing computing power, and since you bought the mac pro in the first place, maybe mobility isn't as important.
is keeping the mac pro and saving for a MBP of some kind an option?
As the above poster said, the Mac Pro is worth more than the Macbook Pro. Personally, I would never "trade" a computer with someone, even if they were a close friend. You never know the history of the computer you're getting, and if they screw up the computer you trade to them they might blame you for trading them a faulty computer.
If you really want a Macbook Pro I would suggest selling your Mac Pro and using that money to buy one.
The new i5 MBPS apparently out perform the 2.8ghz C2D of yesteryear. So yeah, I would hold on to the Mac Pro and either get a cheaper MacBook that you know will be slower or wait to get a newer corei series MBP.
Maybe if you really need to be portable, and the Mac Pro does not make sense anymore for you. Selling separately would be better, or some cash needs to come along with the MBP.
only benefit is portability. so if your not looking to travel or lounge around your house with a computer. then the trade is horrible. a mac pro compared to a mbp is like a bmw 3 series to a bmw M3. both nice, but the mac pro is a notch or 2 better (prob more) but anyways, stay with the mac pro!