One of the GRG regulars raised the suggestion that perhaps we don't see increased cancer risk in mice following telomerase therapy - and that's after the research group in question dropped the addition of tinkering with P53, and moved to straight telomerase overexpression, interestingly - because the immune system improvement means that there is more and better cancer immunosurveillance going on. I'm not aware of any earnest data to support or contradict that hypothesis, however.