I attend several conferences per year, and most of them there are people to whom it would be useful to communicate the existence of math.SE. Does SE produce some kind of promotional material? The one thing I have in mind is something like a nice and attractive poster one could give to organizers to display at some visible place, &c.
We certainly do! In fact, we sponsor our community users to go to conferences of their professions or interests. We also create swag and moderator cards too.
An example of moderator cards. I'm also thinking about creating generic site cards too(without personal information), so non-mod users can request them and pass them around to their friends and colleagues.
I know we have a post on the Math.se swag design idea, but it doesn't seem to have a clear winner yet.
For conferences, I think it may be a good idea to list all the major upcoming conferences and plan ahead of time. That way, we can send swag directly to the conference location so our users don't have to carry them. Or you can pick ONE big conference and iron out some details. For example, our WordPress community's recent sponsorship proposal.