I just tried what you said and here are some facts: apparently when I use the #2 on the top of the keyboard that happens, but when I press 2 on the NumPad it will allow you to select ok after pressing M. I am using 2.49 on XP.
Sounds like you’re htting the two button and it’s not going into the “move box” but instead is switching you over to layer two and leaving the cube in layer one? can you click the second layer and it still works? I’m guessing you’re on Windows?