I haven’t seen anyone calculate the odds of McCain’s lasting through his first term. What follows is a crude estimate using life tables from the Social Security administration. They do not take into account sex, class, and other relevant factors. The result is undoubtedly an overestimate, but it gives some sense of the magnitude of the probabilities we’re considering.
Probability of dying within the next year if you're 72:
.033
If you're 76:
.048
Odds of living through the four years from 72 to 76:
.857 (i.e. a 1 in 6 chance of not making it)
Odds of living through the eight years from 72 to 80:
.683 (i.e. a 1 in 3 chance of not making it)
The numbers for Obama:
4 years: 0.980
8 years: 0.954
The data are from 2003 Social Security tables, not broken down by sex or income or other relevant factors.
Addendum: I’m talking about conditional probabilities, of course. The probability of a President Palin given that Obama is elected is undoubtedly much smaller.