I just spent a couple of hours helping a friend de-bug a WordPress 3.0 failure on one of her websites. The tags functionality on her posts stopped working after she had upgraded to WordPress 3.0. It turns out there is a compatibility issue between WordPress 3.0 and the Headspace 2 plugin at this time (June, 2010). So if you are experiencing a plugin problem with your posts no longer allowing your to add or edit tags and you are using this plugin, deactivate it. I ‘m certain there will be a new version with a bug fix posted soon.
Meanwhile, you may want to read my posts on safe upgrades and troubleshooting.