This question often arises also in the promotion issues, since faculty in other sciences (esp. biological and medical sciences) and humanities have a very different approach to listing the co-authors. I am not totally sure, but I think mathematics is more of an exception than the rule in this respect for whatever cultural reasons. This "ordering" problem is especially difficult in CS and applied math departments, I think, which have both people who order according to the research contribution, and people who use alphabetical order.
In any event, in the last few stages of the promotion approval, when the candidacy goes to the faculty senate, president/chancellor, board, etc. there is often the need to explain this. To clarify this issue, the chairs/deans often write guidelines where they explain the conventional alphabetical order, which sometimes become a part of the faculty rules. My quick search returned these faculty rules: at UGA and UMN. So in the event you need to make a promotion case at your institution, you can use these as a "supporting evidence".
P.S. National Academy of Sciences also voiced these concerns here (search for "alphabetical").
