Convoy, the 2007 Eastercon, has been cancelled. There’s a brief statement on the website:
The committee of the 2007 Eastercon regrets to announce that Convoy cannot now be held at the Adelphi, and that membership fees will be reimbursed by the beginning of December 2006 to those who had joined the convention.
But that’s all anybody seems to know at the moment. The guests of honour for Convoy were going to be Judith Clute, Peter Dickinson, Robin McKinley, and Sharyn November.
vector-bsfa.com 
Ah, that sucks. I was looking forward to going to my second Eastercon, too, this year’s was a blast.
What odds of something smaller turning up as a stop-gap?
I think there’s going to be lots of talk about what happens instead at the Tun, at Novacon, and in other places. That may result in the “something smaller” that the Armchair Anarchist is talking about — but finding a venue of decent size that’s still available over the Easter weekend at this short notice will be difficult.
Other questions arise about events that “always happen at Eastercon” — such as the various award ceremonies (including the BSFA awards), and a bid session for the 2009 Eastercon.