I am retracting my previous retraction of my statement that 40K is a greater problem than 14C (circa 21 Oct 1999). I've updated my calculations to reflect a 14C abundance of 1 in 10^-12 C atoms from a recent paper by Robert Freitas on the subject. For details see:
This isn't the final word on the problem. Robert Fs' draft paper on the subject suggests that we are good for 500-1000 years on ice but still needs some work, as does, I suspect, my brief commentary.
The bottom line for now is that 40K and 14C are both of concern but are probably not of "significant" concern (given the progress in biotech/nanotech that we can reasonably expect).