I managed to check the GLSL version, and it's in fact 1.30. I've started a separate thread asking whether it's possible to set the GLSL version explicitly. I've had no trouble compiling GLSL 3.30 shaders in a FreeGLUT-based tutorial I've been following.

Also, I've compiled my code with Qt 4.8, and I get a triangle even when I request OpenGL version 3.3. I've checked the format in initializeGL(), and the version is actually 3.3. So it seems something funny is going on with Qt 5.2 (or something funny is going on in my code, and Qt 5.2 is less tolerant).