Nice article. You do a good job of outlining the issues that come up on the database side of things when doing Agile development.

The one thing I'd add is that we support the Agile methodology, but we also come around behind (underneath, whatever) the Agile process and do regular database only performance and stress testing, independent of the iteration cycles. Because of this we know that the designs that we're constantly tweaking and adjusting are remaining a viable design.

I absolutely agree with your final point.