Blizzard's announced a 24-hour downtime window for a large group of servers, from 12:01 AM Pacific time on Tuesday, June 8th, to 12:01 Pacific time on Wednesday, June 9th. All other servers will have a normal downtime window of 5 AM to 11 AM Pacific time.
The extended downtime, says Blizzard, is to prepare the servers for the Cataclysm expansion. While this should not be taken as a sign that the expansion is imminent, it's at least nice to see that preparations are continuing apace. In addition, it's probably fair to assume that even if your server isn't in this current list, it'll eventually get a day of downtime of its own as Blizzard gets all of their servers in shape.
Check the full list after the break to see if your server is in this batch: