While it's true there's no official release date posted by Blizz, the large retailer's release dates are very telling and can be used as a guide. While local stores might speculate, the larger dealers work closely with marketing staff of the vendor (in this case Blizzard) and normally require a vendor to give a fairly accurate projection. The longer the advance preorder period the more variance that is normally allowed in the actual delivery date. Normally when the time frame gets to about six months the retailers request a date that is accurate withing about 30 days. When I preorderd in October 2007, Amazon was listing a delivery date of the first week in December. I was notified last month that the new delivery date has been adjusted to the first week in November. This should indicate that a resonable expectation is a publish date of October/November.

IMO there is no real reason to wait to order unless you want to hear what others think about the expansion first. If you want to play as soon as possible you should go ahead and preorder. Check with the vendor you are going to order from to make sure, but most don't charge your credit card until they are ready to ship and you can cancel the order anytime up to the time they ship. If too many people wait to pre-order and then order when the game is released there is a high possibilty that Blizz will not create enough copies in the first run and some people will end up having to wait a month for more copies to be released.