I know TW2 is a pretty high-end game, but I don't think it's supposed to heat my GPU to over 100 degrees Celsius. That's with the fan running at 100% intensity.

The GPU I'm running is an AMD HD6850. The case is that of a Dell XPS 420. When running most games the GPU get's to around 75C, but TW2 alone brings it into the 90's and above.

I've tried dusting out the inside of my computer with compressed air, under-clocking the GPU a bit with Catalyst Control Center, turning up the fan, and turning down the settings in the game, but none of it makes any difference. Is there any other good way to keep temps down?

Have you got any fans in your case? It sounds like you need to pump the hot air out of the case.

I dont know what settings you are running the game but this game played on high or ultra push my 580 to maximum, and comparing the temps of the Witcher 2 with for exemp. Hard Reset i get higher temps with the Witcher 2 more or less 5º
to 6ºC more but fully loaded my system never goes more than 70º im running a a ASUS ENGTX580 DCII/2DIS/1536MD5 on Corsair 650D box.100ºC is too much and you will end up burning the V.card.If you are getting the same temps with different settings theres something wrong.Dont forget to disable ubersample on the game settings.

I've tried turning the game down to medium but it hasn't made any difference at all. I just didn't expect this game to heat the GPU a full 20C above most other games. All I can see are the GPU fan the PSU fan, and a heat sink in the center.
This is what it looks like inside: http://i.imgur.com/FdmDD.jpg (I've tidied up the cables since then)

I get crazy-high temperatures along with huge framerate dips in certain places, like while meditating. The menu is almost unusable because of this.

Well it sounds like a problem I had when my case fans died. Try keeping the panel off when playing, if it drops to a more stable temperature than that's likely your problem.