Or, better yet, create a 'geo' field that will work with maps and sync between Mac and iPhone:

http://www.bioneural.net/2009/02/09/add-a-geo-field-to-iphone-and-address-book/

Not sure how to get that to work on the iPhone with OS3.0, as the share location option doesn't exist anymore. It's been replaced with a share contact option, which send a vcf file. So I don't see any way to get the coordinates on the iPhone.

Of course it does still work if you create everything in OS X then sync I guess (haven't tested it yet).

The Share Location button appears if you search for an address in the Google Map app, rather than using a Contact.

To work around it, go to your Contact's address in Google Map. Once you have it displayed go to the address bar, and change it slightly (removing the Country worked for me), touch Search. Now it will display the pin as an address rather than the contact. Now touching the (>) button, will show the Share Location button at the bottom instead of the Share Contact button.

Hope this helps!