Look Ma, no DB!
A friend forwarded a link to the Prevalayer project. Although it has been out there a while, I hadn't run across it before.
Basically, it offers a system for transparent persistence of objects in a variety of OO languages. I tried the C# flavour called Bamboo on a few simple cases and it worked as advertised. For me at least, it is a solution in search of a problem, but it is a very cool solution.