The IP address should be enough - you could post it. The person who has
that address could then announce themselves the winner of the contest. But
should the winner get a chicken dinner from Petra or a steak dinner from
Don. What do you think - double or nothing. If Don's Kalispell forecast beats
out Petra's Helena forecast, then Don has to come up with a way to celebrate
the 100,000th visitor, if Petra wins, she has to do it.
Ideas from Petra, Don (and others) would be welcome. We can control
when that 100,000th visitor arrives by writing programs to access the site
every 10 minutes, for example, but I don't know any of the people who worked
on the McDonald's Monopoly game, so I guess I'm not shady enough to
do that. We'll just have to rely on random chance.
If nothing interesting happens in the next week, the winner will probably
not occur until September.

Now here is another way of doing this, that will not compromise anyone's
internet privacy. Let's guess the time and date that the 100,000th visitor
arrives. The winner is the closest guess.

I'll be first, how about:
The 100,000th visitor will arrive at this site at 3:00 p.m. Tuesday, September 4, 2001

That's my guess and I'm sticking to it (for now)