Not surprising given the incredible overwhelming number of people trying to play the game. I knew it would be huge, I never imagined it would be the phenomenon it is. (Go to a park at lunch or after work if you don't know what I'm talking about. Any park in the country.)

Especially that they launched US-only (?), but the whole world is playing anyway thanks to game's APK being spread on-line. I'm writing this from Poland, coming straight from 2 hours walk from work, catching Pokemon and leveling up on the way :).

It first launched in Australia, and is available in a few countries right now (Australia/New Zealand/USA). I believe the rollout to the UK/Asia/other countries is on hold due to issues with the huge volume of traffic overwhelming Niantic's servers.

Seems like this would be very easy for them to block since they know your GPS coordinates. They could just pop up a message saying "Pokemon Go is not yet available in your region," although, the good will demonstrated by allowing customers in other countries to play would probably be better than arbitrarily blocking them.

They could. If they did, they'd probably kill the game on the spot, or at least significantly curtail its popularity. As it is right now, the game is a global phenomenon, and it's getting immensely popular worldwide.

Being a semi-Google company, maybe the server code is written in Python, that's why the servers are always overloaded 8-)

Being the guys who wrote Ingress, I'm betting they still host their stuff on Google App Engine... ;).