The geo:
scheme, being relatively new, isn't supported by any builtin apps on iOS. (It's a good idea for a feature request, though.)
One alternative (supported all the way back to iPhone OS 1.0) is to use a Google Maps URL. For example: http://maps.google.com/maps?ll=37.331044,-122.029202 -- this will get automatically redirected to the Maps application. (I dunno if or how this might change with the forthcoming non-Google-backed Maps in iOS 6, though.)
It's also possible to register a third-party app to handle URL schemes not used by system apps, so you could create your own geo
handler if that's useful to you.