Out-of-the-box, Midgard supports multiple location sources: user-entered, Plazes, Dopplr, Fireeagle and so forth. We could also do browser location quite easily.

I wrote a plan how to do location-sharing services on maemo.org way back in 2007, but the possibilities have moved forward quite a lot since. I'll check if that can be dug up from mothballs and modernized. But anyway, keep the ideas rolling in...