It depends on how the url was originally formatted.  If you were using urlformat="HumanFriendly" and the Url looks something like http://www.dotnetnuke.com/OldPage.aspx then it will not return the home page, and give you a real 404.

If you had it like http://www.dotnetnuke.com/tabid/555/Default.aspx then you will get a 302 (temporary redirect) to the home page followed by a 200 (status ok).

Obviously the last one is bad for SEO, but at least your visitors never get an ugly 404 page.