The URL format is very easy for directions:
http://maps.google.com/maps?saddr=[source address or coordinates]&daddr=[destination address or coordinates]
Here is an example of taking the coordinates pulled from Core Location, and using that as the starting point for directions:
NSString *destinationString = @"Cupertino,California";
NSString *url = ;
Of course, this doing this quits your application and launches Maps.app, so make sure you've got everything saved before you do it.
There is one gotcha if you're using search terms or a physical address instead of coordinates. Maps.app doesn't like spaces. Unlike Safari, Maps.app won't automatically convert spaces to %20 for you. On the other hand, you don't want to use NSString's stringByAddingPercentEscapesUsingEncoding: either, because that method will convert commas, dashes, and certain other characters, which may cause problems when launching Maps.app (and probably will - those commas are important!).
In most cases, you can just remove the spaces by doing this:
NSString *newAddressPartOfURL =
Technically speaking, we're cheating. We should be able to URL-encode the whole address no problem, and it should work. But… it doesn't. This is one of those situations where you need to cheat a little to get things to work properly. I can tell you from first-hand experience that if you URL-encode those commas in your address, you will get a message that Maps.app couldn't find the specified address.