This is a simple web page you can use to find out if your browser supports the HTML5 geolocation API.
I've hosted it at http://neo-geo.gilesb.com.
Heavily based on code from Merge Design's blog.
I used this to test my iPod Touch for geolocation (it works). Because Neo Geo shows you your longitude and latitude, you can set it up on a server, walk around with your phone hitting reload, and find out what level of resolution your phone's geolocation has. In fact, if you do exactly that, I'll be very curious to find out what results you get; this exists to gather precisely that information.