As someone who actually has experience chairing a postdoctoral hiring committee in math in the US (maybe F'x has? s/he is an international wo/man of mystery...), I think the answer is "it's unclear."
I think there's no question that mentioning your spousal situation in connection with a job will increase the probability of you both getting jobs and decrease probability of either of you getting jobs; it's impossible to be sure how those things will balance. There's an extremely real danger that if a school knows about your situation, they'll just decide they have no chance of getting you and give up; on the other hand, if a school has a reasonable number of positions (for example, the University of Michigan), and you and your spouse are reasonably hot commodities, there is some chance a school will try to grab you both, but only if they know before whatever crucial meeting they have. That's not a very likely turn of events, but it has happened (note the importance of at least one, and hopefully both, of you being a hot commodity, and of the school in question having a large number of positions; if they have only one position, this won't work very well).
In part, you have to decide for yourself how scared you are of not getting a job vs. how scared you are having to take a job separating yourself from your spouse. No one on this site can tell you, but maybe your advisor or another trusted mentor can help you work that out. If you're planning for a research career in mathematics, you should also give some thought to doing the best postdocs possible (maybe requiring separation) to have better leverage on the TT market (I know that sucks, but the two-body problem generally sucks).
If you decide you would like the committee to know about your spouse, you then have to figure out when and how to tell them. I wouldn't depend on anything being noticed in your cover letter; there are a lot to read, and such a subtle thing will go unnoticed. As other answers point out, there is some chance of coming off as presumptuous. It's probably more reliable for one of your letter writers to mention it (bonus: this requires you to have a discussion with the letter writer about your situation). You can also broach the subject with someone you know at the school via email (or ask your advisor to do this). Another useful trick is to include a link to your spouse's homepage on your homepage (though that could be easily missed as well; still I'm more likely to read a homepage than a cover letter).