Microsoft just posted this note on their newsgroups:

Thanks to the posts from the few of you that were having trouble getting build 2800, we were able to discover that one of the servers in our distribution system didn't take the propogation of the new build correctly, thus leaving 2799 and causing anyone that happened to hit that particular server to still see/get the old build. This should now be corrected. If anyone is still getting 2799 when they try to download, please let us know!

Download Windows Media Player 9 Series Beta (for Windows XP) - updated build 2800