There will be a series of rolling restarts Wednesday morning for all U.S. realms beginning at 5:00 a.m. PST / 8:00 a.m. EST and lasting a "minimal amount of time." That usually means about 15 minutes for each realm, and the restarts are usually completely over with by 7:00 a.m. PST / 10:00 a.m. EST.
There is no information as to what the restarts fix, however things have been relatively stable tonight and there haven't been any abnormal number of complaints that WoW Insider has picked up on.
And since you'll obviously be up at five in the morning and will need something to do, why not check out our patch 3.1 coverage (hopefully the PTR will drop very very soon), or read up on dual specs? Good times ahead to be had by all.