[XorNot]

Well if you consider the effect progress has on birthrates (they drop - below replacement rate in fact) then it seems likely that we have effectively unlimited time to delay having children, it's likely birth rates would get even lower.

However, clinical immortality isn't absolute - an on average we would survive about 250 years between fatal accidents.

One of the conclusions I've always thought we can draw from this is that it's a decent explanation of the apparent paucity of life zipping around the galaxy: civilizations discover immortality before they get warp travel or whatever, their birthrate drops to near zero and the population stabilizes so you get no exponential growth across the stars, and so relatively few of them are actually out exploring.