I may be able to help with a config change. Can you email me (just reply to the forum notification when you get it), and I'll send a config file for you to try. This will just be a simple 301 redirect for the default.aspx?base url.
Incidentally, I think this comes from something to do with splash screens. When you set a splash page for your site, I believe the DNN logo would set up it's linked Url as /default.aspx?base
-Bruce