Ossian Studios' Tiberius209 paid a visit to the official Neverwinter Nights 2 forums in order to bring us the latest details on Mysteries of Westgate's release.
OK, this message will be split into information I know and what I think. The first is definite. The second is just my own (slightly informed) opinion. Do not take any of the second as a "promise."
What I know is that some distribution sites were ready last Friday. Others needed more time. Once everything is finalized and ready to go, there are a series of final OKs that need to be received before any systems can be brought on-line. These OKs come from more than just Atari and are expected to take very little time to get. Once all the OKs are received, Atari will put out the official press release.
What I think is that all the systems are almost certainly ready by this point and that Atari is only waiting on the final go-ahead, if that is not already received. When the press-release comes, I believe there will be no further delay until it can be bought. In other words, the press release will say something like "available now," but they might give a 1-2 day heads' up. I still have a funny feeling this Friday will be the release day.