I agree with Ryan. 301-redirects to mydomain.com would be the best solution since index.html is the default file on your web server and visitors prefer remembering mydomain.com in stead of mydomain.com/default.html. Besides, unless "default" is a keyword, "default.html" doesn't have any seo value. In fact, it's recommended to use folders in stead of file names.
You can change the default page in your web server's apache configuration however.