Plugin Compatibility – As mentioned above, many plugins will no longer work in 2.1 due to deprecation of database table variables. However, whenever there is a new version of WordPress, the other possible “plugin breaks†extend farther. Fortunately, the WordPress community has been hard at work testing plugins with 2.1 determining compatibility. You can find a list of backwards-compatible plugins at the Codex.
Source: 10 Things You should Know About WordPress 2.1 » Technology, Blogging and New Media
Last week I updated all of my WordPress blogs to 2.0.7 even though I knew 2.1 version was coming soon. There were security issues and Mark’s changed files update minimized my work on this minor update. As you can see, this update is a bit more complicated since I need to check my plugins first.