ergophobe - 4:04 pm on Jun 22, 2010 (gmt 0)
SK - you're referring to the merge with WPMU I assume?
I haven't looked at WPMU in a long time, so not sure this is absolutely right, but my memory is that WPMU requires you to use one DB for all sites.
So in terms of "update once" for WP3, the problem is that if you currently have a lot of standalone WP installs, it's not going to suddenly roll them into one database and therefore can't update in one operation.
FYI - in the Drupal world, I found the multi-site function to be a major hassle unless your sites are all using the same base theme and plugins. What happens is you have one module that doesn't get ported to the new version and so all of your other sites are stuck using an older version until that one module upgrades. I eventually sucked it up and split the sites to use separate code bases.
I'm not sure how that compares in the WP world.