I have just updated a wordpress from 2.7.1 (spanish) to 2.8.4 (english) and get some problems on redirecting from login page to control panel.
I deleted all files and reinstalled catalan version that is updated to 2.8.4 version, and now everything works fine.
I think that the problem was on upload files to directory, I simply overwrited files and maybe some one did not upload correctly (this is not first time that happens to me something like this).
Would be great that next installs will check that all files in directory are the files on lastest version. Maybe inserting a comment on each file or simply checking the date and time of the files and advice you that some files were not up to date (showing your plugins and themes folders).
My two cents.
Thanks for this great project!