Both the Mac OSX Address Book and Contacts/ Phone apps on the Apple iPhone support a custom "geo" field. You can use this field to store GPS coordinates that will open a Google Map when right-clicked on Mac or tapped on iPhone. The reverse geocoding in Google Maps isn't always perfect; this gives you the option to store a more accurate location alongside a human-readable address.
Creating a geo field in iPhone Contacts
If you have an existing contact with address details in the iPhone Contacts app, tap on the address to launch Maps:

Maps will place a pin at the approximate location of the address for that contact:

Tap the blue and white arrow on the tag to see the Info screen, then tap the Share Location button at the bottom left:

In the mail message body note the GPS coordinate values and jot these down:

Go back into Contacts and edit the address to which you want to add the coordinates. Tap the plus icon to add a new address:

Tap the address type button at the bottom right of the next screen:

Now select geo as the address type:

Enter the coordinates you jotted down earlier into the first field:

After editing the coordinates should display underneath the existing address:

Tapping these coordinates on the iPhone will launch Maps (although you may notice odd pin labels—not sure why that is). After sync you should see the corresponding geo field in Address Book under Mac OSX:

If you don't have an existing Contact address you can of course start with Maps and fine-tune the placement of a dropped pin. You can create a new Contact from within Maps, obtaining the coordinates by sharing as above:

Creating a geo field in Address Book
Say you've opened a geotagged photo in Preview and clicked on Locate to open Google Maps. You'll see a coordinate pair in the info window like this:

Select and copy the coordinates from the search field (this seems more reliable) and create a new contact in Address Book. Use the address-type pop-down menu to create a custom address label 'geo':

Paste in the coordinates you copied earlier and save:
You can right-click on the geo field title to open a Google Map on your desktop:

Alternatively after you sync you can access the coordinates on iPhone:

On iPhone the only downside is that you may need to edit the location field once Maps opens (e.g. to remove the "United Kingdom" which Maps insists on adding during reverse geocoding):












Just a situation when you could do with copy-paste in the iPhone :(
Is 2.2.1 software needed to display "geo"-field ?
In my iPhone 3G 2.2 there are only "home", "work" and "other" options.
Well this post is dated 2/09 and according to iTunes I didn't download the 2.2.1 update until 3/09, so I don't think that's it Kino. Not sure why you don't see the geo label but perhaps try adding it as a custom label?
Ok, then it must be because I'm syncing my contacts with GMail contacts, not with OSX Address Book.
Great find. Very useful.
Regarding having to remove that pesky "United Kingdom" (or in my case, United States)...
The country is just another data field, and can be changed. Unfortunately, it cannot be changed on the iPhone through the built-in contacts app.
Using ContactsTool, which allows LDIF export and import of contact data: I first changed the country of the "geo" address on the iPhone to something else using the iPhone's contact application. Downloading the file, a new field suddenly appeared: "ctoolCustomAddressc" which held the country. Uploading a new file, I verified I could cram whatever I wanted into it.
The trick, then, is simply finding something that Google Maps will ignore. Fortunately, that's easy! If you include text in parens after a set of coordinates, Google Maps displays that as the pin label.
So: change the country on the iPhone to something besides your default country, download the contact, change the country to read "(Something Useful)", upload it, and import the contact. You'll wind up with another contact that you CAN click on, and it will go right to that point in Google Maps, even labeling it for you.