if new iMacs are being released it will go down closer to 4:00am pacific time, like it did last week when they included Final Cut Server.
it's believed when the store goes down closer to Midnight it's for routine maintenance, whereas when the store goes down in the early morning hours (4am-6am PST), they are releasing something, as it generally has been historically, but not always.
When they updated the MacBook Pro/MacBook in february, and MacPro / XServe in January, the store went down earlier (Midnight - 1:00am PST) and it was down for 4+ hours. perhaps for both maintenance as well as updating... but no one really knows.
about a month ago the store was down nearly all night, around 6 or 7 hours, and nothing at all was released, and there were no new store design changes...
if the store goes down tonight, check to see if all the stores have gone down besides the US store... if they've all gone down, then they're releasing new iMacs, MacMinis, Displays, or whatever hardware that is sold worldwide expecting an update. if it's just the US/Can stores, or the UK/France/Germany stores, it's going to be either something very very dull (like Final Cut Server now shipping!) or routine maintenance.
also, it feels very quiet on the boards tonight - not many posts or replies... feels like the calm before a storm