Marshall et al,
It's a lack of IP Address Space - and the numbers I gave - 10's of thousands are probably a bit on the small side - in short order it will be multiples of 100,000 IP addresses. To start with, I'm willing to think in terms of 10's of thousands spread over a handful of "POPs". The application is GPRS (aka 2.5/3G cellular) and each Internet connected user or some major subset of them will likely wind up with an address on their mobile device. - JB