1. I'm using 1.5 stable.
2. I'm sure my web server has turned on URL Rewrite in my directory.
3. The ".htaccess" file IS writeable (chmod 666).
4. I clicked the "Update Permalink Structure" button many times, it shows "Permalink structure updated." with NO error. But actually, the time ".htaccess" last changed NEVER changes.
5. So, I guess WordPress never update the file truely but reports the WRONG thing.
I wonder whether there is a solution, thanks :)