Clint, I've done something similar although most of my rocks were glued flat versus "standing". I've always loved the look of the standing rock planter pots but they're SO expensive.
Here's the link for the method I tried:
http://70.32.92.162/Pebble-Mos.... I used Thinset Mortar to attach the rocks and it worked great, especially since it didn't set up as fast as the adhesive caulk the article suggests. The major problem I see with their directions is they didn't recommend sealing the terra cotta pot first? I sealed the outside of mine with a primer paint, an added plus was the primed surface gave a rougher surface for the Thinset to "grab". However, one year later I'm discovering cracking in the some of the grout between the pebbles and realize I should have sealed the interior of the pot as well.
I think they'd actually be okay if you removed the soil over the winter and brought them inside. Unfortunately I forgot all about them and I think the wet soil freezing and thawing is causing the problems.
FYI, if you don't like the look of the grouted surface you can always decide not to finish with grout. The technique I've used to get as deep a groove as possible between the stones is to grout the whole thing and then use a toothbrush to remove the excess grout.
Just make sure if you do decide to use Thinset mortar and/or grout, to use a
penetrating sealer. Other sealers will provide protection but just form a coat on the surface and not "bond" properly.
Clint, I hope you will consider posting a step by step tutorial ... or maybe write an article with a tutorial. 
Think Dave might want to update the intro? 
