Yeah, there's a problem with the SDK. It's affecting a variety of people (myself included) but I think it's limited to those who own ATI graphics cards (although not all ATI users are affected). At the moment, there's no definate fix. To help with the problem, try forcing Hammer to launch in DX8 mode. This can be done by making a shortcut to Hammer, going into the Properties of the shortcut and adding "-dxlevel80" in the Target field. So the target field will look similar to this:
"C:Program FilesValveSteamSteamAppsant292sourcesdkbinhammer.exe" -dxlevel80
This should help, but it won't fix it completely. At the moment, models, the SDK models and selection boxes are affected.