I found a bug.

I have an ATI card and an nVidia card in the same computer. GUIMiner only looks at the set of devices from platform 0, but with two such cards, the platform also needs to be specified. In my case, the ATI card is under --platform 0, and the nVidia card is --platform 1. So, GUIMiner should support both platform and device selection. Those with only one platform would not worry about this, but those with two or more platforms would not have trouble.

I should also point out that with more than one platform installed, GUIMiner refuses to work at all, so I'm using poclbm on its own.