I found this question to be very relevant to me. I am joining Facebook (presumably about a year before its IPO), and I feel a little of what the top answerer mentioned. I know it's flawed reasoning, but I could have graduated a year early from my university... so I sort of beat myself for it. I'm wondering if others have similar experiences and may want to share?
A friend of mine turned down a job offer from Google in 2000. He later joined Google in early 2004. He regrets not having joined Google in 2000, but he's happy that he did join in 2004 because it worked out better for him financially than the other jobs he considered at the time.
:( That's really unfortunate, but I guess it's hard to know in 2000 what you're getting into.