The cat's out of the bag that 2.0 stable will be available Wednesday or Thursday, so this thread is meant as a preparation for the release. The single biggest problem when upgrading is if something goes wrong, which invariably does, as all servers are not built the same, the user failed to backup. So we are encouraging all WP users, especially those who will want to install the new version as soon as it is out, to back up all files before the release. This means not just your WP directory, but your database as well. The database contains all of your posts, comments, pages, etc, and if anything goes wrong, and know that major changes to the database occur in 2.0, you could loose all this data. This can not be stressed enough.
Also note before upgrading, that major changes to the way users/authors/etc are handled. So if your blog has multiple authors, you may want to completely familiarize yourself with those changes before making the switch.
This includes ANY plugin that relies on user level. It would be highly advised to deactivate those plugins before the switch.
Just a final note, though the team of developers do a fantastic job of smashing bugs, working out kinks, etc, things can go wrong. There will be very detailed instructions posted with the announcement, and sometimes, even following them a glitch can occur. There will be volunteers here to assist, so be patient, provide all relevant information, and trust they will do their best to get your blog back up and running.
I'm sure more will be added, but please, there's is no exact time of release, so don't ask.