So, I just found this, and found it to be repeatable, so I'd thought I'd share. I picked up Dogmeat, did some stuff, then went to Point Lookout. When I went to go get Dogmeat back, I found that if you sneak up on him undetected, you can pickpocket him (you receive bad karma). So I gave him an assortment of used weapons I might need for repairs when I come back to pick him up again. You can only pickpocket him when he's waiting at Vault 101 for you.

That "feature" was introduced with Broken Steel. A word of advice: Don't try to store explosives in him

Aww, poor dogmeat becomes hotdog meat.

Not a very good feature..... if you can access him at all.

You could also use the Dogmeat Packmule Mod. As far as I can tell, it does not conflict with other mods and is useful if you don't want to be overencumbered when switching from a good companion to evil companion. Extra storage space (2 companions + player house).

Link: http://www.fallout3nexus.com/downloads/file.php?id=2706