New users just install it like a regular plugin; users of the old version need to undertake an annoying upgrade procedure. (Sorry!)
As with the previous version, there are some things you need to do to enable pingback and trackback. The plugin page has more details.
Sorry this is so late; and worse: sorry it hasn't been rigorously tested. I did try, really, but there's always something I miss, like accidentally deleting the whole database. You won't know until you try, though, and I won't know unless you tell me. :) Bug reports are welcome, but please provide as much information as possible.
Pingback and trackback are entirely untested, because I can't get them to work on my test server with an unmodified WordPress install, let alone a hacked up one. It's a mystery.
RSD won't work, because the link is hard-coded. Does anyone even use it?
The most important change since Multiply for WordPress 1.5 is that alternate blogs are no longer counted up from #1. The first blog created with the plugin used have an ID of 1, the next 2 etc., but this conflicts with the new $blog_id variable -- WordPress 2.0 uses it in the core, and by default it's 1.
To avoid this, the upgrade script will, in an incredibly dodgy manoeuvre, re-number the old blog #1 to something else. If you have blogs numbered one through three, #1 will become #4. Please note, though, that the $mb_press_id variables in the index.php and xmlrpc-$x.php files you may have created will not be changed automatically.
A system of user roles and capabilities has replaced the old user level system, and it's multi-blog compatibible out-of-the-box. For you, dear user, the effect is that the Multiply admin page no longer contains any user functions at all.
Mostly, this is a very good thing: you just switch blogs, go to the user page, and edit the user levels. The downside is that you have to do this with every blog. User #1 is added as an administrator by default; no other users are added at all, but they should default to 'subscriber' level.