Whew!
Turns out you just need to add an IIS 7 specific section to your web.config file to get Blogengine back in the land of the living.
Gory details here: http://www.codeplex.com/blogengine/Thread/View.aspx?ThreadId=21124
Glad it was an easy fix because I don't know how long it will take to get my software to the self-hosting point, and it would suck to lose the links to this blog.