Hi,

   I found that for proper highlighting of the tabs, it is not necessary to add the filename like default.aspx to the link if the site is in the same sitecollection. When you type in the link, it won't highlight, but if you use the 'browse' button next to the link field, which presents the 'Select a link -- Web page dialog' and browse for any available site or page, that will result in proper highlighting.

In order to browse to other sites in the collection, you need to click the 'up' button in that dialog a few times.