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 “<Alt>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.