There're only a few windows I can't minimize with Alt+Space,N. One of them is googletalk because it doesn't use actual windows or something. GoogleTalks's got all sorts of non-standard behavior.
But Putty I expected better from. They allow Alt+Tab but not Alt+Space.
This reminds of when I discovered that GLUT eats Alt+F4. I had to put handles in for GLUT_KEY_F4 when (glutGetModifiers() & GLUT_ACTIVE_ALT) and then exit(0).