How high can you dry stack a raised garden bed and still fill it with soil, without the bricks/stones toppling over?
A skilled stone mason can probably build a dry stacked wall as high as you want it. For all others, I'd say keep it low enough to avoid a safety hazard, dig down and start the wall below grade a bit using the largest stones, and slope it in slightly toward the soil side so it doesn't fall outward.
I'm not a mason, but since no one else piped up...you can probably get a lot of info on the Stone forum.
Thanks for directing us to the Stone forum. We hadn't found that one yet. I also posted this question to the landscape design forum, and there are some good responses, as well as a follow up picture of what we did this past weekend: http://forums2.gardenweb.com/forums/load/design/msg0417250119248.html.
We did start the stones a bit below grade. We also canted them ever so slightly toward the soil side. This weekend, on the advice of our local, independent garden center, we covered the inside with surface bonding cement. It feels pretty solid, now.