How GPS savvy are they? I would almost bet you are mistaken... Its got to be there somewhere. I have noticed with automotive GPS, finding the menu for inputing coordinates can be hard to find. Even if there really isn't a menu option to load coordinates, you could still load it through the computer. I use EasyGPS and love it. Free, user friendly and quick!
I've got a Garmin Streetpilot 500, and as far as I know you can't input coords, and I'm pretty savy. I prefer the Streetpilot for navigation because of it's ease to use such as touchscreen, voice prompts etc. I use the 60CSx for caching.
El Diablo
I'm not sure if they changed it but I have the Garmin Streetpilot 330 and it takes coords. I just send them straight to the unit through the Geocaching website. They go to my favorites. Our if you have Mapsource you can put them in that way as well.