Michael Silverbane
Adventurer
I'm not sure "GM decides" covers the same ground though as "Mother May I".
The GM decides a lot of things in general, and so it makes the terminology a bit vague.
When someone uses "Mother May I", I'm thinking of a style of play in which the DM explicitly blocks the players from taking certain actions, or determines the outcome in such a way as to render player-actions useless. It is the direct opposite of a DM that says "roll the dice", thus allowing just about any action, but using the dice to resolve the outcome.
For example, I played in a Star Wars RPG, where the DM told me that I couldn't try to pilot a stationary X-wing in a hangar. He didn't ask for a check, even though my character had a pilot skill. That would be an example of "Mother May I" in my opinion. He could have asked me to make a skill check (not "Mother May I"), or he could have just set the DC impossibly high (soft "Mother May I", but almost just as bad). What he did not do, is allow me to just take the action and/or have a fair chance at pulling it off.
But please correct me if I'm interpreting the phrase incorrectly.
So, to me... It seems like Mother May I is the degenerate case of GM Decides.
A game where the GM has Veto power on player action declarations is GM Decides. When that veto power causes game dysfunction (such as in your example above) it becomes Mother May I.
The amount of veto power usage that will cause Mother May I will vary from table to table.