Wow, it's been over a month since I've posted...  Just a quick note this time, I re-read today "The Vietnam of Computer Science" by Ted Neward from 2006.  This is certianly one of the best blog posts ever on technical development and the definitive explanation of what the "Object-Relational Impedance Mismatch" is really all about. 

While I generally agree with what is said in this article, I think that most of the problems mentioned can be readily addressed just byadopting a slightly more pragmatic perspective.  The exception is inheritance-based design patterns which simply have no good answer in the relational model.  Yes, there are answers, but they all have significant drawbacks.

If you've never read it, do yourself a favor and read it now.  If you have read it before, that's no reason not to read it again...