Like the others, I will ocassionaly take 2, rigged and ready rods with me. It is a hassle, but if I break the rod down into 4 pieces, it will fit in the back, vest pouch....sorta.
Like Midger says, the line gets mucked up, but it's quicker to leave it rigged, than not. I'll take a rod for dries and nymphs, and one for pulling streamers.
I have seen a rod holster to carry a spare rod on you back. Haven't found it on a quick Google, but I have definitely seen such a thing.
Like Richard, if i'm in a watercraft of some sort, I carry a minimum of 2 rods. Usually 3, and sometimes 4 rigged and ready. Floater, intermediate, type 3, 4, or 6, and type 8+ to dredge.
briansII