Let's see what we know so far:
- As of January 5, the next wing of Icecrown Citadel will be unlocked in two weeks, or around January 19.
- If the final wing is released 2 weeks later, that would be February 2. I'm going to say February 9, just because it's Arthas and I wouldn't be surprised if Blizzard drags it out.
- The current arena season ends on January 19th. It started on September 1, so it lasted 4.5 months. If the new season starts right away and runs the same amount of time, it would end sometime in late May.
- The Wrath of the Lich King beta started July 17th, 2008, and WotLK released November 13, so you're looking at approximately 3-4 months between the start of beta and the launch date.
- To add to that, patch 3.0.2 (the launch event patch with the barbershops, achievement system, Stormwind Harbor, Inscription, and all the other WotLK goodies) released on October 14, a full month before the launch date.
- As of right now, there have been no reliable leaks of a Friends & Family Alpha of Cataclysm, nor any word on the Beta. This seems to put a February/March release date out of the picture, as there likely wouldn't be enough time to complete testing.
So given all of that:
If we hear about the Beta soon, say, sometime in late January/early February, and then allow 3 months for testing, we could be seeing the launch date as early as April. That seems really early to me, again, knowing that we're talking about Blizzard here, so I'm thinking May is more likely. I also think May is more likely given that the next arena season may be ending around that time frame, and they could start fresh with both the expansion and the arena season. Plus you have the Memorial Day weekend, which would be prime gaming/buying time. This means that patch 4.0.x, the one that starts the pre-launch event, could very well start in April.
So yes, my guess is May 2010 for Cataclysm launch, with patch 4.0 going live in April.