Why is SSE scalar sqrt(x) slower than rsqrt(x) * x?

I've been profiling some of our core math on an Intel Core Duo, and while looking at various approaches to square root I've noticed something odd: using the SSE scalar operations, it is faster to take a reciprocal square root and multiply it to get the sqrt, than it is to use the native sqrt opcode! I'm testing it with a loop something like: inline float TestSqrtFunction( float in ); void TestFunc