The timing of the iPhone (its OS and its app store ecosystem) played a significant role in the check-in model, I think, as it encouraged developers to [finally] sidestep the carriers, and also prevented background processing. Though this is kinda irrelevant to Chris' bigger point of pivoting towards a perceived advantage.
(It will be interesting to see how things change with OS4's background processing. I'd guess users will continue to prefer for check-in based over continuous gps logging, though there may be some interesting/appealing applications of continuous logging. Haven't really looked into the specifics yet.)
Google Latitude on the Android follows the continuous logging model. It's interesting but for very different reasons than Foursquare. Foursquare's appeal to me is more keeping a record of interesting places I go and sharing that with others. Latitude's appeal is, oh, where are my friends right now.
(It will be interesting to see how things change with OS4's background processing. I'd guess users will continue to prefer for check-in based over continuous gps logging, though there may be some interesting/appealing applications of continuous logging. Haven't really looked into the specifics yet.)