I wanted to write about this topic for a long time, but of course it was a matter of time, or the lack of it.
Nowadays, one may be a post doc at one place (or one country), and move on to another post doc at another place (or another country), and on and on and on. I am not making this up. I have seen it with my own eyes, not just based on some rumors. (I didn't know when I wrote this Blog that there are articles in Nature and Science on this issue!)
If you love to travel around the country or around the world, there is nothing wrong with being a post doc forever as long as you are wanted.
If you have a family and a child, you probably want to settle down before your child starts his high school, which is four years in the U.S. Fine, if you can find a post doc position for about four years and live near a great public school; go for it.
If you want a job, a real job, not just a post doc position, then you need to think hard about your future. Sometimes, it’s better to move on to a different type of job or even to a different field than being stuck in a dungeon...
ps. I revised it a bit today, Nov. 9, 2012 when I started to collect info under "Post docs."