Of course having the servers back online as scheduled was too good to be true. And at 12:09 PM PST, Blizzard announced that maintenance would be extended by an additional four hours in order to deal with issues arising from the patching process. Yes, that means you shouldn't expect to be online and playing before
4:00 PM PST 5:00 PM PST -- at least if we're optimistic enough to assume this will be the afternoon's only hiccup!
Update: Drysc, always trying to outdo me, posted an update by the time I clicked to post this. We are now looking at 5:00 PM PST.
Update 2: All realms should be online and accepting players as of 6:00 PM PST.