Edit: To clarify, the PTR is getting a new patch/build. Patch 3.1 is not hitting live realms today. Sorry for theconfusion.
Dresorull, a member of Blizzard's QA team, has just posted that the PTR is coming down (has come down by now, actually) for maintenance and a new build will be pushed through There's no ETA for when the realms will be coming back up, but it's safe to say that it will be a few hours at least. Dresorull also says they'll update the maintenance thread when they do have an ETA, so that's something we'll be keeping our eyes on.
There's sure to be a ton of changes in the patch, and as soon as we have more information, we'll be sure to pass it on to you. Personally, I'm looking forward to some of the little bugs in the Argent Tournament being cleared up so I can test out the rest of that chain, but I'll take what I can get! Tonight is going to be a busy night at the WoW Insider HQ, I suspect.