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.