I live in Brea, CA. You can't drive around for more than 10 minutes and not see a P-car. Along my street alone, 18 or so homes - three 986 and two 996.
Not common at all. I live in a small rural college town of 20,000(including the school) and I have the only boxster here. No base models or S's. If there are other boxster owners in Charleston Illinois they keep them locked up tight because I haven't seen them. There is however a 996 cabbie owner in the town next to us about 10 miles away in Mattoon IL.
I live in the re-developed South Beach District of San Francisco and in my apartment's garage, which holds about 100 cars, there are currently 4 Boxsters (including mine) and 3 911s alone. There are even a couple of (crazy) owners that park theirs on thesurrounding streets. Here in the Bay Area of Northern California they are very common
As for the countries, I used to work in Frankfurt quite often and there were a lot of Porsches there. As a British citizen though, I can certify there are far more Porsches here in California than in the UK...though this is probably as much due the glorious British weather as to anything else!