I have upgraded this blog to WordPress 3.0 not long after it was available for download. Everything went without any major issues. I only use it without special hacks. One thing that makes this version a little bit different is that WordPress will bring your site offline if there is/are something not working. For example, when you’re upgrading your plugins. WordPress will temporarily bring your site under maintenance mode.
If all process were completed, you should be happy. Otherwise, you will need to do some extra works. WordPress will display this kind of error message: “Briefly unavailable for scheduled maintenance.” You will be locked out from your admin area. Your website will be inaccessible. Solution? Easy. You need to go to login to your webhosting server, and remove .maintenance file. You can find it in your blog folder — the same location with your wp-config.php.