Has anyone used anything like this before? I'm planning a RTW trip in about 18 months and also planning to hike the Appalachian Trail and I'm thinking something like this might be indispensable for those (many) times when you can't remember where the hell that pic you're looking at was taken.

Journal... pen... photo number.

Low tech, but it works.

Yeah, that's definitely the foolproof, quick and dirty way to go.

But ... I'm a gear junkie. And I'm also planning to thru-hike the Appalachian Trail. While it's quite easy to whip out the old pen and paper and note that your photo was taken at the Coliseum or [fill in easily recognizable landmark here], I'm worried it'll be a bit more difficult to note exactly which tree or shrub my photos were taken near while in the woods.

There are several pieces of software out there that will add gps tags to your photos by comparing the timestamp from your photos and your gps. that way you cam buy a more functional GPS

Buy a Nikon D200 and the appropriate cable. GPS coordinates and orientation embedded in image metadata.