Originally Posted by Labmouse
Well, I don't really have any inside industry knowledge but I think I can make a couple of guesses...
The reason GPUs are soldered onto the mainboard is because it saves money and space. Space is at a premium on a laptop, especially anything smaller than the DTR category. The only laptops I've seen with a replaceable GPU module was with big DTR models like the Dell I8600
Also, understand the reason that you won't find really powerful GPUs on thin and light laptops is due to the amount of heat those GPUs put out and the cases they're put in. The latest fastest mobile GPUs from ATI and Nvidia run hot. They run so hot, if they didn't have any kind of cooling, they'd stop working after a few seconds. Trying to get something like that working in a thin and light would be prohibitively expensive, or prohibitively heavy.
(Even the ati 9700-M in my laptop doesn't run for more than 30 seconds without cooling.)
The GPUs aren't soldered on for that reason, because they're obviously upgradeable in the factory. It's not a TDP or space problem, in other words, and it's not a socket problem. Now, I know that there are limitations put in by the power connections already present in the laptop - these won't ever power an X800. That doesn't mean that they won't be capable of powering XI700s, though.