The forum had several problems, which discouraged people from actively using it, so a redesign has gone on, which has definitely improved the general appearance of the site, but seems to have lost (temporarily?) all the old posts - and any chance to start new ones, for the time being.
[Update 7th April 2010]:
In case you don't dig into Comments, I should say the new forum looks terrific, and everything I have tried works fine. So good, that this post now almost counts as an April Fool panic joke.
Many thanks to Clyde, as ever.