A few technical notes:
The netradiant binaries were compiled on Arch Linux, fully updated, Linux 2.6.27 and Gtk 2.14.4. It seems gtk-config
reports a different version and my 32bit machine (which I used to compile the 32bit version) doesn't even have gtk-config.
The error in question does not seem to be Radiant-specific, a bit of googling revealed it pops up in a wide range of programs, and usually related to intel graphics. My guess is that it has to do with the video subsystem.
If you are running a fancy desktop with e.g. compiz you can try disabling it.