Just installed 2.5.1, am using Yahoo Web Hosting. When I try to use the "day and name" setting for permalinks, I get a 404 after a long delay after clicking on the post. If I set it back to "default" it's fine. I'd really like to use a prettier format though. I saw a bug that said this was fixed in 2.5.1, but it still seems to be an issue on mine.