Going from MBP to macbook is a downgrade any way you cut it right now. Sure, you get the nice new design, but you give up dvi, dual display output, firewire, two inches, expresscard, and I believe you'll lose 30-60 minutes battery life. Your Q1-onward 2008 MBP is probably quicker than the new macbook as is, has more vram, is cheaper to upgrade, and it already suppotrs neat stuff like swipe gestures. At $1000 off, I'd get a late MBP now myself instead of the unibody.
Yes, the MB can be viewed as a 'downgrade', but not in all respects. You're mistaken on battery life, vRAM, gestures, and upgrades.
The stated battery life for the MB is 5 hours (compared to the MBP which is 4/4.5). If you consider real world computing, that number drops a bit. Design and trackpad gestures are identical between the two machines. The extra vRAM (only the 2.53Ghz model and up) will NOT translate into faster computing. The card is 128 bit so I'm not sure it can even utilize the full 512MB of vRAM. More importantly, however, until SnowLeopard is released, I doubt you will notice any difference in real world applications. Even when SL is released, there is still no guarantee that the extra 256MB of vRAM will make a noticeable difference (and again, this depends on what you use the computer for). Concerning the cost of upgrades, any savings with the MBP is offset by various 'hidden fees'. By this, I simply mean that you have to pay more for Apple Care, external cases, etc. It's not a huge point, but it's something you should keep in mind.
In the end, I think the deciding factors should be size, weight, and screen quality. You need to ask yourself what you'll be using the computer for and base your decision on that. If you're going to carry the computer around (say, in a backpack), 1lb can mean a world of difference. From a performance perspective, the high-end MB is almost identical to the low-end MBP. In my case, the only tangible sacrifice was the screen quality. Even then, the screen on the MB is arguably better than most. And with the money I saved from 'downgrading', I'm going to buy a kick ass monitor to watch movies on (that will obviously blow the MBP screen out of the water).
Remember, 'downgrade' is a relative term - it depends on what your needs are. The MB can be viewed as a 'downgrade', but the MBP can also be viewed as 'overkill'.