I have found a few geocache/letterbox hybrids in my region (all planted by the same person I believe) and they seem to be serving all interested parties just fine. I like the format and am planning to plant one or two myself.

Some good practices that I've observed with these boxes, and that I plan to follow...
1. They are listed everywhere - AQ, letterboxing.org, geocaching.com, and each listing clearly states the nature of the box.
2. The stamp is well marked "Do Not Trade - Leave in Box" (or something to that effect)
3. The placer pre-stamped several index cards and left them in the box for people to take if they don't have a logbook to stamp in. I especially liked that idea, gives something for people to take without endangering the stamp.
4. There is an info sheet explaining both hobbies.
5. They are well hidden and require a mile or more hike so people really have to put some effort into finding them (though if you're adopting a location you may not have any control over this)

My two cents...

~ Leabhar