A Debian developer recently pointed me at this free C++ library for linear algebra:
http://eigen.tuxfamily.org/index.php?title=FAQ
Basically it is as fast as MKL (on a single thread, multi-threading support is still preliminary), and it has a very nice syntax.
I regret I did not know it when we started implementing the Estimation DLL. I am not sure if it is worth switching now. But we should definitely consider the possibility.
Best,