Open ‘gconf-editor’

Lookup the ‘apps/metacity’ folder.

Open the subfolder “global_keybindings”and set “panel_run_dialog” to “disable”.

Next set (in the same folder), “run_command_1″ to “F2″

Now open (in “apps/metacity”) the subfolder “keybinding_command” and set key “command_1″ to “gmrun”.

Now hitting Alt-F2 should get you gmrun, instead of the stock run dialog.