There have been several startups doing the same sort of thing, all failing. I'm still not convinced people want this (Location aware social networking mobile thingy). It's a shame you can't get traffic figures so easily for mobile apps - without user/revenue numbers it's all speculation as to how successful or otherwise loopt is.
IPO seems like crazy talk to me.
I don't use my cell for networking. I don't like that constant connectivity. But within that market, I would suggest that the Dodgeball team is a force to be reckoned with.
Perhaps I'm biased because Loopt is ugly as sin, and Foursquare is slightly better.
Similarly, youth is a good reason to think that Foursquare could be a threat. The people making Foursquare have already created a successful product in this market. They spent a few years working for a powerful company that didn't understand their ideas. So they're the people who hit the market before Sam did, and they're returning with a more refined concept than they had before. The fact that they're youthful and determined is why I'm watching them and ignoring attempts like Google Latitude, which practically scream sterile.