Sabre is a great plugin, and the author has indicated he feels it is compatible with WP3 in "mono" mode (without Multi Site activated). See this page: http://wordpress.org/support/topic/389051?replies=2
On that same thread, the plugin's developer (Didier) also tells that he is not yet supporting the Multi Site (Network) Feature of WP3.
I was using Sabre in 3.0 before I turned on 3.0's Multi Site (Network) feature. I turned on the Multi Site feature before I knew that Sabre was not compatible, and (honestly) without really considering whether Sabre would be compatible. I just wanted to give you all a heads up:
I created a new site on the network. Then I added my main Admin account as a user on that new site. Not sure if I was supposed to do that (I am real green on this). Anyhow, that action somehow deleted the Admin account's "OK" status in Sabre (it lost its valid user status). I can no longer log in with that account. Luckily I had a backup account with Admin status also, with which I can still log in.
When I click into that main Admin user account, its confirmation status (at the bottom of the page, where the Sabre stuff is shown) shows empty.
Here's where it gets interesting: I tried to manually confirm that account, to get it to be valid again, and Sabre gave me this reply:
WordPress user doesn't need to confirm registration. Manual registration ineffective.
So, somehow Sabre knows it is confirmed (on one hand), but at login time Sabre does not know it is confirmed (on the other hand).
I guess I will try to edit the MySQL table(s) manually to get the account valid again. Any advice on that would be appreciated, but not expected, as I know the developer is not supporting this aspect.