If no one ever died, the world would overpopulate even faster. Death makes life possible. Death clears the old to make way for the new. In Hinduism and Buddhism, accepting death is a part of the path to enlightenment. Personally, I believe the body can die, but consciousness cannot...so we are already immortal, we just have to re-member it.
If nobody ever died, the whole concept of procreation would change profoundly. With first world countries already in zero or negative population growth, creating an infinite lifespan would most likely further that trend.
And in any case, we're talking about longevity, not immortality. You can still get hit by a bus.
As long as we push for women's rights, access to birth control and abortion rights, and move away from economies which encourage large families as sources of labor, we'll be fine, as has already been the case in much of the developed world (and is showing to be the case in the developing world as well).
The stigma around childbirth is a little scary to think about. But all things considered, I think that a society would adapt to limit births by some means if we achieved clinical immortality.
And there are plenty of means to limit birth without stopping sex. Just asking people to sterilize themselves would probably be sufficient, especially since recombinant cloning actually is at least a reasonable thing to talk about with current technology. (Sterilization doesn't mean you can't ever have kids, just that it's going to cost you a fair amount, and you will have a lot of paperwork to do.) The concept of clinical immortality, in contrast, hasn't really advanced beyond the legend of the "fountain of youth."
Also, about a century ago people thought we would face terrible overpopulation by now, due to limited ground space for buildings homes. That's before we learned we could make apartment buildings.
Don't think of the future 100 years from now, with the eyes from 2011. You don't know what conditions might be created by then to allow for increased population. Humans tend to find solutions for everything, especially when approaching a huge crisis.
Enormous cascading effects, and comparing these possibility to past events ... im sorry the scale simply pales in comparison.
Overpopulation is an idea based on scarcity. The technologies which enable immortality will also work to overcome scarcity.