I mentioned that I have the error or warning of the Joomla! 1.5.2 on Dreamhost.
Now I know what happened.
My previous upgrade steps are:
1) Upgrade the Joomla! 1.5.1 through Dreamhost one-click installation.
2) Manually upgrade it to 1.5.2.
So problem comes.
I use this way to upgrade Joomla! 1.5.2 without problems.
1) Restore the .old installation of Joomla! 1.5.0. It works perfect without problems.
2) Download the Upgrade Patch from 1.5.0 to 1.5.2 from official site.
My Joomla! 1.5.2 is OK now. I think the Dreamhost One-Click upgrade of Joomla has some issues.