GUIMiner doesn't work with older versions of the OpenCL drivers. I was having the same issue (Catalyst 10.11 + SDK 2.1 with my 5830).

I uninstalled all the ATI software, and installed just Catalyst 11.6 (which includes OpenCL drivers). Then GUIMiner works.

The 10.10/11+SDK 2.1 method is for the command-line miners (like Diablo) only, not for GUIMiner.