I have a 'Photix GPS' and have used it while travelling overseas, it lived on the D300 the whole trip.There is a model for the D90.
The results are seen with 'Geotagger' free software - a split image of your photo and the location on 'Google Earth'.
Of the 1200 pix taken on the last trip, the ones that had limited time to locate satelites and indoor shots registererd the last plce co-ordinates, but most where in the ballpark for location, even when taken out of a bus.
The time to aquire satelites is about 55 seconds, so the unit is useless for opportunistic shots, but great for planned shots that require some time to shoot. Leaving the camera on all the time resulted in draining two batteries a day.
Getting a good result in cities was a bit hit and miss as the buildings blocked out readings some of the time.
End result - I have welcomed the setup as now when I look at my photos of my trip I can recall where the pics where taken, no problem at all.