I experienced this issue with the Add Media bar above “write post” being absent. At first glance, it appeared as though 2.5 had implausibly done away with image upload!
I was clued in by the comment that said 80% of these issues are resolved by following the instructions provided in the included link. I tried removing wp-admin and wp-includes and replacing them from a freshly downloaded copy of 2.5, and deactivating plugins. This did not help. Then I realized I had only tried firefox. In IE, the Add Media buttons were present. So I cleaned out the cache in Firefox, and that fixed it.
Incidentally, in the Firefox error console, suggested above, there were several critical javascript errors generated by loading the “write post” page (something about Jquery, have forgotten details), which disappeared after clearing the cache.