I was talking to a friend of mine about this who really wants to buy a laptop now, but doesn't want to get burned too bad when new ones roll out. There's definitely a value in not having to compromise your productivity because you want to wait.
I myself just bought a new 15" 2.15ghz macbook pro 2 weeks ago beacuse I couldnt wait and needed to get stuff done. However, I'm in a unique situation in that my wife needs to upgrade her old iBook soon anyways, so my plan is to hand this off to her once the new MacBook Pros roll out and then get the latest greatest for myself. Then I'll be good for the next couple years, while she'll be a little outdated, but its alright, she doesn't do heavy things on the laptop anyways.
anyways, here's my suggestion...
Strictly speaking in terms of MBPs... I would suggest to buy used because an upgrade really appears to be coming in the near future, and its seeming like its going to be a lot more than just an OS upgrade.
Looking at the 15" 2.16ghz, A new one w/ education discount goes for $1799, and the refurb ones from apple (when they're available randomly in the mornings) go for $1599. So if you can pay like $1300ish for a used, I bet you could probably still sell for pretty much what you paid for it once the new upgraded MBPs roll out.
Try to buy from someone who still has enough warranty left to go into late late '07 (of if you're lucky, early early '08) so that when you resell the laptop, you can still sell it under warranty, and the buyer has the option to put on applecare.
I never recommend extended service plans except on apple.