Comment 52 for bug 769644

After much searching, I checked the key-binding section of ~/.config/openbox/lubuntu-rc.xml, and found the command to execute for Alt + F2 was: lxsession-default launcher_manager

I changed that to: lxpanelctl run and now everything is hunky-dory.

Bit staggering to have to get under the hood so early in the piece to spanner a new install, though.