I've mentioned it before, but one of my mandates here is to make sure there is room for, as well the tools necessary to support, a healthy criminal underworld. That includes the sort of infrastructure you might imagine when someone says "theives guild."
An organization of thieves is going to need a way to organize and coordinate their efforts to avoid "overfishing", a way to offload easily identifiable goods, a way to forge papers, and a way to handle "splitting the take" or otherwise distributing the gains of the organization safely.
I'd suggest organizing as an association, perhaps with a public-facing goal of bettering the plight of the poor and homeless within their community. It would justify real estate purchases in strange areas, legitimize the movement of shady characters into and out of association property ("Training pickpockets? My word! No sir! We're educating the troubled youth of this slum!") and provide a mechanism for distributing the wealth ("We're giving out alms, your grace, as you can clearly see").
Of course, now that I've mentioned it... ;)
There's some question as to allowing deviant-only contracts and organizations outside of that, too, but nothing firm at this point. We kind of like the idea of "shadow contracts," but it's not 100% necessary since you can work out the same actions using the existing tools we'll be providing.
Anyway, hope that helps! :)