You can join all of the iPhone and Android people by;

1. Installing the MicroB Geolocation addon from Extras.
2. Login using http://m.gowalla.com

Gowalla is MUCH prettier than foursquare, and actually insists on checking where you are from the GPS before allowing you to "check-in".

It seems to not cache your login (or at least that is what I was seeing).
Also, it looks like it sends login details using only HTTP
Oh, and this has to be the heaviest mobile version of a site EVER