I just have this problem, and I don't remember I changed anything before this happened.
If I browse to my blog http://tentangcad.com, it will always redirect me to http://tentangcad.com/default.html
I tried to check at my domain and DNS settings, the .htaccess file, nothing changed. And nothing mention about 301 permanent redirect.
Anyone knows about this? I believe this is not WordPress specific, because I also have problem with PhpBB on the same domain, different subdomain.
I hope you can help :(