Yeah… I have the same issue. I’m really thinking its a WP 2.8 issue. I’m running Leopard, using Safari 4 (Final) with the WebKit nightly builds and Firefox 3.0.8. It does the same thing in Ubuntu, though.
When I try to add a category or tag and click on either box for the tag name, slug, or description, the field disappears (so I can’t enter any text) and it looks as if the blank field appears on the right side (as if I saved the tag/category). Its really odd.
Oh well. Its not a big deal for me so far…
Oh… BTW… in the ‘Add new Post’ section, I can add tags or categories… its only in the specific Tag & Category sections with the AJAX that have problems.