I have been having no ends of problems with the new WordPress 2.8 as it relates to saving Post Tags. I can type them in, but they won’t save when I click the Save Draft or Publish buttons. Below is a graphic of the area I am talking about.
What I came to realize is that there were some plugins that were causing the problem. So, I simply deactivated all of them and then re-activated them one at a time. Before re-activating any of them though, I tried saving Post Tags and, lo and behold, it worked! So I re-activated a few of the ones I needed and disregarded the rest. Now everything is working fine. So if you ever have problems saving Post Tags, or even a Post itself, just deactivate all of your plugins and then re-activate them one at a time until you find out which one is the culprit.