After weighing things up, and particularly with the advice of Greg Bray in the comments there, I'd pretty much decided on the i7, so on 2 November I was ready to buy. I went to the Dell shop... and discovered that they'd added GBP400 or so onto the price for both models in the spec that I wanted. Welcome to the pre-Christmas price rise.
So I waited until early January, and finally the price for the i7 returned to where it had been (though the T9700 was still expensive). And last Thursday, the new machine arrived. i7 720QM quad-core at 1.6GHz (up to 2.8GHz when only one core is active), 4Gb RAM, ATI Mobility Radeon HD 4670 with 1Gb RAM (which can do 432 GigaFLOPS[!]), Windows 7 Pro 64 bit, 256Gb SSD, and a 16" RGBLED screen (if I understand that correctly, the backlight is an array of LEDs in groups of 3, and it adjusts the backlight colour in a screen segment to match the colour in that part of the screen). All for the same price as the cheapest 15" MacBook Pro.
It's lovely. The build quality is excellent -- the only comparable machines I've tried have been the MacBook Pros and the Air, which are nice but slower and considerably more expensive. (To be fair to Apple, the MacBook Pros are better-made than this Dell. But it's a close thing, and I don't think it's worth the extra. YMMV.) The screen is the best I've seen since I saw high-end CRTs on SGI Workstations back in the 90s.
Size and weight-wise, it's comparable to a MacBook 17" -- smaller, but in the same category. So I wouldn't recommend you get one as a netbook -- but as a desktop replacement, it really looks like it's going to be perfect.
The only thing that sucks is the face recognition login widget, which has so far managed to recognise me once. Out of 30 tries. But hey, it can be uninstalled.