Do graduate schools in mathematics typically send out acceptance letters all at once? There have been a few grad programs that have sent out acceptances to many people - in particular, Ohio State, UCLA, and GA Tech. 

I am wondering if this means that these schools are done accepting students (with the exception of wait-listed students).
The short answer is "no" or maybe "it depends what you mean by "wait list." " 

Take a moment to think about how graduate admissions works: departments have a relatively fixed number of slots that they have money for, and it is a serious nuisance if you go much above or much below those numbers.  However, because of how the deadlines are structured (with everyone having April 15th), you essentially have to make all your admissions decisions having very little idea of how many people will turn you down (because it's rational for them to wait until April 15th).  In my experience, the admission committee will do one round of admission letters that they hope is enough to fill their class, but which is probably a little on the conservative side. Right now would seem a little early to me (many schools haven't even finished their winter break), so it's possible you're seeing people who are getting early ones for some reason.  Even after that, it's still quite possible that they will decide to admit more people if for whatever reason they decide that they are not likely to get enough from their initial round.  Those admissions could happen basically any time; I've seen it happen once on the morning of April 15th.  

Whether the pool of people who might still get admitted actually receive a letter saying that they are on the waitlist is mostly up to random bureaucratic chance, but if you haven't received a rejection yet, and someone else has an acceptance, you should regard yourself as on the waitlist.