Most of these startups treat the iPhone as if it were some magical device that's plugged straight into their API. I actually participated on a Get Satisfaction thread for Gowalla where one person claimed they liked that it was iPhone only (which dates how long ago it was) because it made it "secure" and couldn't be "hacked the way Foursquare can."
Needless to say, they were disabused of that notion pretty quickly when checked in at the Gug, the Apple flagship store in London, and my home within the space of 30 seconds. It literally took 3 minutes to figure out - most of which was spent setting up my iPhone to proxy through my laptop. They were (and keep in mind, this is back when Gowalla was still iPhone only) sending everything in clear text.
They also didn't regenerate session IDs for anything at the time. I logged out and logged back in, same session ID. I logged out via my iPhone, loaded up their site and changed my password, then logged back in, same session ID. Basically, once you had sniffed one session ID you were set. I tried it a week later and was still able to use the same session ID without any problem.
It's amazing that things like session hijacking are so unknown and unfamiliar to people people relying on web frameworks to do everything for them. At the time they were using Merb---I found out by hitting an "/unknown/and/unknowable/" page and seeing their backtrace output. Session hijacking is a solved problem, why they were subject to it I can't fathom.