This explanation gets a bit into themeing, which isn't well documented yet, but here goes.
The "Hot" and "New" default menu items both use "hot.php" in the default theme (because the "New" page is a template of "Hot"). The "hot.php" page delivers stories one of two way - either ranked or time-sorted, depending on the URL. Currently it only recognizes "hot" and "new" as valid URLs.
So to use a different default, you need to change the record in the "ns_settings_site" table to the new default URL (for example, "top"), create a new page in the admin section with this URL and select "hot" as the page template, and modify "hot.php" to recognize the new URL.
Alternatively, you could modify the record as above, create a new page with that URL, and create a "top.php" page in your template to handle it.
Either way will work. A place to set the default landing page will be provided in v0.7, so you won't need to edit the database directly then.
Let me know if I can help with this or explain it better :)