I'll more than likely buy one of the new Nehalem-based Pros, but my rationale is not based on brute power now, but on future-proofing. I'm no expert on Intel's development roadmap, but my understanding is that Nehalem is a fundamental architecture change, which suggests that we're unlikely to see a further step jump like this for some years. Snow Leopard will bring in better multi-core support, and developers will start to optimise software for multiple cores over the next few years.
As far as I can see it, buying a Nehalem system might be paying a premium, but may keep pace with software development a year or so longer than a current Harpertown-based system, so in the long run represents more bang for buck. I'm happy to have this assertion disproved by someone, but my philosophy is generally spend up front to maximise long-term benefit.
The graphics card option is woeful however- putting a £60 graphics card in a £2500 machine is like putting plastic bucket seats in a Ferrari- I'll be holding off on my purchase until the 4870 option is in place.