I am stumped. I upgraded from 10.04 to 10.10 Beta 3 and for some reason I can't get Alt-F2 to do anything. Is this a known bug or am I just not doing the right thing?
This is a known limitation of the new Unity shell. The bug to track is here: https://bugs.launchpad.net/unity/+bug/580295
It is a known bug: https://bugs.launchpad.net/ubuntu/+source/gtk+2.0/+bug/398826
Like the others said, Alt-F2 is gone in netbook-edition 10.10 because of the switch to Unity. And as other suggested, gnome-do is a good replacement.
However, there is one more important step to make it work. The default keyboard shortcut for the Gnome-do menu, Super + Spacebar (aka Win key + space), won't work because Unity grabs use of the Super key. SO, after installing gnome-do you'll need to modify this. You can do this by running gnome-do from a terminal and using the drop down preference menu to modify the shortcut. I like Ctl Alt Space.