We've been hearing an awful lot about Ulduar and patch 3.1 lately. As such, this announcement is really not a surprise. Vaneras and Bornakk posted on the forums today to warn us all that Season 5 is drawing near an end. The current season should close at the same time 3.1 drops, or very shortly afterward. (I'd guess "very shortly afterward" means "the following Tuesday.") Your Arena points will reset with the season transition, though your Honor points will remain banked.
The announcement doesn't set a specific date. He just says "soon." Since we traditionally get at least two weeks' warning before an official season ends, that does mean that the patch probably won't drop for at least three. And since Noblegarden has been expanded on the PTR, then I expect the patch to hit and season to wrap up in the beginning of April.
We haven't heard details about season-end rewards yet. That's important because we don't demonstrably know whether those rewards are going to be based on personal rating, team rating, or even the Match-Making Rating. The details of season-end rewards tend to fluctuate a little, but the best Arena teams can probably expect a title. The very, very best can expect the Arena mount. However, we don't know if it'll be a Nether Drake or maybe even a Proto-Drake. We'll let you know as we find out more.