I’ve just now noticed this bug in Blender 2.60.0 (r41226) where I’m unable to enter Edit mode in the Solid shading mode, while using either MultiTexture or GLSL view. I can oddly enter Edit mode in Textured or Wireframe mode, however. This problem does not occur when trying to edit another object in the same .blend file; perhaps the model I was working with is corrupted now, somehow.
The model has a handful of shapekeys applied, as well.
Before I noticed this bug, I had an armature selected in Pose mode while I was weight painting a mesh. Normally, I’d be able to hit Tab to switch to edit mode for the mesh, but this no longer works and causes the application to semi-freeze up and stay in what ever mode it was before (either Object to Weight Paint mode).
I’m not able to orbit the viewport in real time while the application is “locked”, but the changes take effect when I switch back out of Edit Mode.
This may just be an error with my nVidia graphics card altogether. The first time I tried making changes in Edit Mode, an error message from nVidia appeared telling me that Blender must be closed. The second time, my graphics card crashed.
Edit: The graphics card is NVIDIA GeForce GTS 450
Edit #2: Removing all but 1 Material applied to the object seems to let me be able to enter Edit Mode. This is pretty confusing to me now.