Maybe it is me but my 'moral antenna' says it wrong to leave users suddenly stranded.
Yes it was a 'free service', but the free users made it possible to get millions in investments. Users investest time and effort into making your service valuable.
Here is how I would do things differently.
Depending on the cost, I will have a shut down period of 3-6 months. Longer if possible.
I will make it easy for users to export AND import their data into a similar service. In this case 4sq. That is the very least they should do. The data from a users Gowalla account is ppractically useless if it is not put in context of a similar app/service. Telling millions of users "take your shit out by next month, thanks for your time" is not cool at all.
FriendFeed did it much better. Etacts is the worst I have come accross.
It is not cool for Gowalla, it will also not be cool for gmail.