$900 for a early 2008 Penryn MBP is a steal. You can probably sell it on ebay for $1200 with buy-it-now easily.
It is a downgrade to go from a classic penryn MBP to a high end unibody MB. While it may look "cooler" (if you like the whole black bezel look), if size is not a concern, there is no reason why you should shell out MORE money to buy a less capable laptop.
unibody macbook benefits:
1. New trackpad
2. New chipset - DDR3 ram / 1066 FSB
3. Smaller/lighter
classic penryn MBP benefits:
1. Much better screen
2. Discrete 8600m 256mb graphics - about twice as fast as the 9400m
3. DDR2 memory is dirt cheap.
4. Larger screen/ higher resolution
5. Firewire - if you need it.
Performance wise, a 2.4ghz MBP is nearly identical to a 2.4ghz MB. They are both 45nm penryn processors (IIRC) - the only difference being that the unibody MB use the new centrino 2 chipsets that have DDR3 and 1066mhz FSB. While DDR3 and 1066mhz sound like a fancy upgrade, in real world performance (again, IIRC), the difference on average is less than 5% (for CPU intensive apps). That's not a speed increase you would notice.
If we're talking about GPU, the MBP's 8600gt 256mb discrete graphics is FAR superior than the integrated 9400m found in the unibody MB. Real world tests show nearly twice the performance.
Aside from the CPU and GPU, people often overlook the importance of the HD. Putting an Intel SLC SSD will make your laptop "feel" faster than any small processor bump could ever give you.
Finally, the screen. If you're not really picky, then it is not really a big deal - but it is worth noting the screen on a MBP is significantly superior than that on the MB. Even those who like the ultra glossy look of the glass screen cannot dispute that the quality of screen in the unibody MB is just rubbish.
Even if you could do a straight trade from your classic MBP to the high end unibody MB, I would still recommend the classic MBP over the unibody MB. If you have to sell your MBP AND spend another $600+ to get a MB, that is just plain silly and nonsensical.
There is no reason why you should sell your MBP and have to pay significantly more money for an overall less robust system. I say you should wait ATLEAST until the new Nahalem MB/MBP are introduce before you even consider selling your MBP for a MB.