Not really anything to worry about.... after a tournament game (and hand shaking) it is customary to reset the clocks and set the board up. Players then hand in score sheets (sometimes signed by both players) before finding the analysis room to discuss the game. Complications may develop if these circumstances arise during a time scramble, however, one usually has the required piece in hand (Queen or upside-down Rook) when promotion becomes obvious.
Queening

True. True. Forgot about the resetting the pieces formality.
I suppose if you have a "by the rules stickler" opponant, he/she could make you actually promote to the required piece before conceding, but I agree, only in a time scramble with the "winning" player having only a few seconds, could a real complication occur.

I'm not an expert where the rules are concerned... they keep changing! so I took a look at the Fide Handbook, specifically "The Laws of Chess" (2007), and they say this:
Article 4: The act of moving the pieces
. .
. .
4.6 c
in the case of the promotion of a pawn, when the pawn has been removed from the chessboard and the player`s hand has released the new piece after placing it on the promotion square. If the player has released from his hand the pawn that has reached the promotion square, the move is not yet made, but the player no longer has the right to play the pawn to another square.
Last week I was playing a friend of mine in which a situation occurred where I won by checkmate with a pawn promotion (either a queen or rook would have done the trick).
To finish the game, I touched the square with the pawn, announced "mate", and we shook hands.
But then I got to thinking, as is my tendency. Had this been a tournament, and had I not been paying attention, could my opponent have hoped that I left the table and let my time dwindle down, with the pawn resting on the 8th rank, begging to be promoted.
I say, "yes" because I do not believe the move is complete until a player either announces or grabs and replaces the piece being promoted.
On the other hand, is the opponent admitting defeat by shaking hands.
I know I am being technical and surely I will remove all doubt if this actually occurs in a tournament, but I am just curious what would have happened in a tournament setting, had this occurred against a wily opponent.