WordPress 3.0 integrated everything that MU always did so it’s better to consolidate the forums too. There’s a MultiSite forum on WordPress.org and any queries about upgrading or problems using WordPress 3.0 should go there.
The old forums aren’t going anywhere but later today or perhaps tomorrow I’ll be making them read-only. You’ll still be able to browse the forum, and Google will still index the wealth of information already there but posting will be disabled.
So, thank you to everyone who contributed to the forum over the years. You helped make WordPress MU great.