Within reason I'm in the same boat and I'm keeping my 2008 15" non-unibody LED 2.4GHz MBP for the time being.
The beyond obvious factors are:
1. do you need the portability?
2. do you need the power?
I want more power, and I value the portability, but... I "want" a Mac Pro more than I need it right now, and I'm not that thrilled with the prices on the Mac Pro lineup.
I'm kind of waiting to see if Snow Leopard forces a slight realignment of the Mac Pro lineup (lose the 2.26 and shift some prices around)
If you do a lot of heavy stuff - games, 3d rendering, video rendering and compressing, audio processing and you want a lot of storage options, then the desktop starts to come into it's own.
If you are just a more normal computer user, some of this, some of that, occassionally render and compress things and would probably use the portablilty... then the laptop.
You really need to look at what you do and how you want to do it...
The Mac Book Pro is quite powerful for most things, and if you'd use the portability, then the choice is more clear.
The Mac Pro is very powerful, but you can't move it easily.
I'm either waiting for a Mac Pro realignment (I'd keep my 2008 MBP for portability?) or the Nehalem/Arrandale? MBPs later this year.