None of this is available now, but a resource screen found in iPhoto '09 shows some interesting possible-future technology that may let you geotag your photos even if the camera didn't have a GPS module.
The trick, if we're interpreting Tidbits's findings correctly, is that the iPhone may (in the future) have a location application that tracks where you are at all times. By associating the timestamp of when your photo was taken to the timestamp in your phone—which has the related location information—you can then geotag your photos after you've synced them to iPhoto.
Or, the resource could mean something completely different. And nobody knows whether or not this is gonna happen, especially since using the GPS or location features on the iPhone drains its battery fairly rapidly even in the background, but it's one cool potential use. [Tidbits via Ars Technica via Engadget]