vietknight: I have the same problem and I currently have the latest version (2.6.2)
When I enter the correct username and password, NOTHING happens. I just remain on the wp-login.php page. If I enter a wrong username or password, the error message appears.
This happened after I moved hosting to MediaTemple . Is anyone else that has this problem on MediaTemple?
The thing is that I have another WP install in the same account, a different domain... and I can login perfectly on that one. I moved that one exactly the same from the old hosting account too.
There aren't any error logs, no errors, simply nothing. I have uploaded all the 2.6.2 files again, files are exactly as in the original download, nothing corrupted.