"This brings up an interesting dilemma for Foursquare: do they abandon some of the fun, quirky things that made the service what it is, in an attempt to go mainstream?"
I doubt it's a dilemma for Foursquare at all--that's like asking Mark Zuckerberg if abandoning the privacy concerns of Facebook users is a dilemma for him. Hell no it's not a dilemma if it means he's going to be a billionaire. I think early adopters are fooling ourselves (I include myself in this category) if we think for one minute that "nerd-only" services like Foursquare are worried about selling out. Why else would you spend money and time creating something like Foursquare if you're not hoping to eventually be faced with the "dilemma" of being a sell-out and alienating your early adopter/ nerd purists?
This is the new business model: count on early adopters to use and promote your service for free until it reaches mainstream status, then cash in.