I added this to the wiki, but I'll post it here as well.
If you've finished the particular thieves guild city influence quest in a hold, you can pickpocket with complete impunity aside from a 12 gold fine each time you get caught. If you're looking to level Pickpocket quickly you don't need the Sibbi Black-Briar exploit in order to do so.
In my case I just went into sneak (this works even while detected), stole everything not nailed down and when I got caught I just told the guard I was with the thieves guild, paid my 12 gold fine and went right back to doing it, there was no penalty otherwise for getting caught.
You'll need to be careful doing this though, as you can cause some total carnage in a city this way. In Riften, 3 innocents died because they accidentally struck guards when trying to attack me for theft. Balimund, Keerava and the woman that sells food in the market all lost their lives this way.
Spoiler
For a little depth to the game that I really appreciated, once Balimund died his apprentice took over and told me that he was his apprentice until he died and he'll never be as good.
I went from level 17 to 100 in about an hour using the thief stone.

