This, from Reader's Digest:
There are numerous studies that attest to the stress-relieving benefits of pets. In one analysis researchers evaluated the heart health of 240 couples, half of whom owned a pet. Those couples with pets had significantly lower heart rates and blood pressure levels when exposed to stressors than the couples who did not have pets. In fact, the pets worked even better at buffering stress than the spouses did.
I have to admit, I didn't see this coming. I do think, however, that this is more about dogs than cats. I can't imagine a cat that didn't raise tension/blood pressure.