I forgot to mention that your result is only based on Houdini's performance against Rybka, not other engines. For an overall, general measure of strength, you need several different opponents.

However, the time and resources required to prove such a claim make it virtually untestable.

Not really ...

There is little doubt in my mind that Houdini 1.03 has surpassed Rybka 4 in all areas except perhaps SMP efficiency where she might still hold a slight edge on monster hardware at long time controls. However, the time and resources required to prove such a claim make it virtually untestable ...