I agree with lukulus that it would be more logical when the mate would be checked (and counted) first before the Ice Age occurs, although his example isn't a proof that it's actually handled this way. As far as I can see there is no such proof in the database. I looked at all games which end with a mate after Black's 20th (40th, 60th, ...) move, but all games I've found are by direct contact with the white King or by a Knight's move (see link list below). This is not a positive proof that Ice Age occurs first, because maybe no one yet dared to try to test the behaviour ;) The other positive proof would be a game that continues after a would-be-mate after Black's 40th move, but that is hard to find in the database because in such a game the move wouldn't even be marked as a check, so one would have to go through all games with at least 41, 61, etc. moves. So we should simply wait for the one with the inside knowledge: one short post by Fencer would be enough: "Mate" or "Ice Age"? :-)

20th move
Ronin - talen314
El Cid - Pioneer54
Nuno Reis - pauloaguia

40th move
Franklin - volor
yoyudax - asdf
Jaymom - chudu1980

60th move
diogenysos - basplund
Brainwave - Jatack

P.S.: Well, after looking at all those games once more, in each of them actually an Ice Age occured although it's already mate. Maybe that's an indication that Ice Age occurs first?

kleineme: I'd be much more inclined to ask the game's inventor (or leading players, if the inventor is not available), rather than basing anything on the actual implementation here at BK.  There are things here that could stand improvement.

I'm a variant lover but not an ice age player, so feel free to discount my opinion.  But I'd much rather see the rules of what constitutes "mate" remaining consistent throughout the game, no matter what the current move number happens to be.  Let a move be completed before an ice age occurs.

The "taking the king" argument leads one astray.  If you follow that "logic", a king could move into some checks, secure in the knowledge that an ice age was about to save him.  I'm sure we don't want that.

wetware:
I agree ice age should be after black's 40th move. If that is not the case there is an inconsistency between being black and white as it is definately after the white 40th move. Playing an unrated game following the moves of the game in question and varying at the 40th move would determine what actually happens on bk.