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[...] say in march or April the new models are out and you buy the cheapest mbp again. That's £2000 on 2 Mbps.

I suspect most people would resell the old one instead of keeping it, so if they sell it for £800 or so, then they've spend perhaps £200 additional to have a few months of ownership of the current model. Better yet, having today's current model gives a better ability to sit on the sidelines a while and allow for any early production issues to shake out. Maybe even wait until new systems ship with Lion. Might end up costing perhaps £300 differential, but if it buys six months that's decent value.

What will you be able to do with the next gen model that you can't do with the current one that justifies spending another £999?

Therein lies the key question -- I'd venture to guess that for many people the real-world differences won't be particularly noticeable.
 
You are assuming that the everyone is going to like the next model more than the current model. While that is probably true, there is no guarantee.

Actually I'm not assuming it, cause I've already made my mind up as to when to buy: when the new model and Diablo 3s specs comes out, I'll see if it can run it. If it can't I'm buying the current model (if they're still in stock somewhere by that time).

I'm simply saying that it isn't that long to wait (probably) and at least see the specs of the new one before buying a current. But of course, if one needs a MBP right this minute then buy one.
 
type in on wikipedia with inter core 2 and got to successors and this is what you get yes some core 2 procs get discontinued but some may actually get rebranded as i3 procs

http://en.wikipedia.org/wiki/Intel_Core_2#Successors

Successors

Main article: Nehalem (microarchitecture)

The successors to the Core 2 brand are a set of Nehalem microarchitecture based processors called Core i3, i5, and i7. Core i7 was officially launched on November 17, 2008 as a family of three quad-core processor desktop models, further models started appearing throughout 2009.

With the launch of 32 nm processors in the upcoming months, Intel has scheduled to discontinue some Atom, Celeron, Pentium, Core 2, and even Core i7 models. The Core 2 Quad Q8200, Q8200S, Q9400, and Q9400S are scheduled to be discontinued in 2010.[9] Intel may also rebadge some Core 2 processors in the E7xxx, E8xxx, Q8xxx, Q9xxx and Wolfdale series as Core i3 processors, together with new Core i3 processors using the 45 nm Nehalem microarchitecture.[10][11]
 
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