I doubt they would make this a small tuesday release. They would definitely wait the extra week for WWDC for publicity reasons. They want to show it integrated with the iPhone.
Also, even if they are waiting for WWDC, .Mac is not going to be down for a week. Many people rely on that for their email and stuff and apple wouldn't just throw them in front of a bus like that.
If anything, I think they may be testing some code or slowly updating the backbone. They may have messed something up and had to pull .Mac down for a small amount of time.
This is just my opinion though...