Right, WordPress 2.9.2 just came out with a security fix and I've merged the changes into MU.
You can grab the latest code from the zip file at the end of this page but I haven't tagged 2.9.2, so this isn't final code.
Most of the changes are minor, but this release may have the code from 12140. That makes signup and creation of new blogs a lot faster by cutting the number of queries from around 700 to about 368. I really really need people to test this though. If I can't get some positive feedback about it it'll have to wait until WordPress 3.0, which is months away.
So, please test it on a dev server, or if you're really brave and have backups made, throw it on a public server if you're already running 2.9.1.1.
Thanks!