Hi Simon,
Google does not have a problem with 3 urls in this case because if you view the source, we have added a connonical link which tells google the true page url. If you view this site for example using http://www.mojoportal.com or http:/www.mojoportal.com/Default.aspx or http://www.mojoportal.com/home.aspx , if you view the source ofthe rendered page, in all 3 cases, you will see in the header this:
<link rel='canonical' href='http://www.mojoportal.com/home.aspx' />
this clears up any confusion about multiple urls, google will see all 3 as the same page and in search results the canonical url will be shown.
Default.aspx is the actual handler for all 3 of those urls and all page listed in the menu. Since Default.aspx is configured as the default document in IIS, a request for http://www.mojoportal.com is the same as if the user entered http://www.mojoportal.com/Default.aspx
It will resolve to the first page in your site hierarchy which in many cases will be home.aspx but it could be anything, you can move any page to the root of your site and the root requests will resolve to that page. But the page has a name and an url, in this case on this site that page is named Home and has the url home.aspx which is the official url for the page
Similarly but differently, in the forums we have urls like:
http://www.mojoportal.com/Forums/Thread.aspx?thread=3046&mid=34&pageid=5&ItemID=5&pagenumber=1
a user may enter those params in a different sequence or someone may link to it with those params in different sequence, but we have a caninical url in the page which clears up any ambiguity by always having the params in the same order regarldess of how they may appear in the actual url. So if you view the source of the page you see this:
<link rel='canonical' href='http://www.mojoportal.com/Forums/Thread.aspx?pageid=5&mid=34&ItemID=5&thread=3046&pagenumber=1' />
Hope it helps,
Joe