I sympathise with those of you with family living far away. My ds1 has been in California for 11 years now. He married a Californian girl and they have two children, aged 3yrs and an 8-week-old. We are fortunate in that we have been to see the new baby already and are going again next month but every time we leave, or they leave us, the thought is always 'When will we see them next?'.
Oddly enough, for the past four years we have in fact been with my son either on or close to his birthday, which has been lovely. I doubt we shall ever be there to share the GC's birthdays, though. The 3yo is now becoming more interested in Skype and it is quite funny to see how he doesn't bat an eyelid when we sometimes turn up in his house - he is completely familiar with us.
In some ways I have been fortunate in that I almost didn't have time to miss ds1 when he first moved to America as I still had children at home (still do!) and we moved abroad ourselves for a few years so life was busy for us anyway. How we will manage as we get older is something I occasionally think about but I then park it somewhere out of the way when I can't find a solution.