imho
That 3.06 in the MBP is there for the hard core gamers and ex-overclockers (like myself) and those doing professional video encoding where the additional speed can pay for itself over time. Also note: there is no way to do any overclocking on the MBP that I'm aware of. zdnet did put out an overclocking tool a while back, but it only works on the Mac Pro.
While the 3.06 will hold a higher resale value, you'll never get that $270 back that you paid for it. You'll never get that $300 back that they're charging for the upgrade to the 2.8 either.
The thought of a machine lasting longer with the better processor in it, doesn't hold nearly as much water today as it did in times past. That whole reasoning in years past was due to the speed of CPU's doubling every 12 months. Those days have been behind us since about 2003. That's when I overclocked my first chip to 4 Ghz. We haven't seen software developers increase the minimum speed requirements related to CPU power in several years now. Not sure that I've seen any software that requires anything above a 1.8-2.0Ghz chip. And until Intel or AMD can bust through the 4 Ghz barrier, this is unlikely to change.
The thought in buying a computer today should be to get the one that will do the jobs you need to accomplish.
For the op, personally think the middle of the road 2.66 is the way to go if you're needing one right now. Your money would be better spent on that upgrade to a 7200 RPM drive and you'll see more speed improvement there than you will in the upgrade to the 2.8 chip. Personally, I'd upgrade it myself. But, if you don't want the hassle, it's not as big a ripoff pre-ordering the upgrade as it use to be.