A slight variation along the same lines is x >>> 0, which differs in that it yields positive values for values between 2^31 and 2^32-1 inclusive, effectively acting as a C cast to uint32_t instead of int32_t. It runs in roughly the same time as the other bitwise truncation idioms on Firefox 15 alpha, with Math.floor being slightly faster than them.
I am impressed. I thought you were surely wrong, then I checked it in Firebug, then I checked it in the ECMA-262 standard, and yes, a >>> b is indeed a uint32 operator.
