MartinFowler made mention of this in his blog today. Neo is a object/relational mapping layer for .NET. Looks pretty cool. (I can't find any specific mention of open sourcing, but they have a sf.net project, so it must be to be hosted there, though they don't mention which license they use).
I started building a small something like this on a previous project -- it was pretty straightforward stuff for ‘lil ol’ me, so I'm excited to see what some focused effort from the ThoughtWorks guys has produced.
The Neo page also mentions Microsoft's upcoming obj/rel stuffs called ObjectSpaces which should also be interesting. To me, this is one of the next important bits the big frameworks (like .NET) are missing -- it's the only thing I spent the most time on my last .NET project building.