Personally I see no reason for a thieving-skill. Those who play fairly, always give something. I for my part let it depend on my chars history and stats. You could have stolen Baileys shoes without her recognizing. Yet Caitlin, with perception 18 and being a former thief would probably already notice when you only think thievery (j/k.) On the other hand she had a weakness for thieves and just let them do and pretended to not be aware of it.
But I'm getting off topic...
If a thievery skill is really, really wanted I thought of following (*prepares for the beating*) :
Why not giving a skill for good rp. Just for a change. Lets say there is a limited amount of 1-3. Now you write an application with your char concept (char description, background, how you would play the char.) and from those they get picked. Now if we are running dry on thieves again due to any reasons (PO doesn't play the char anymore, PO doesn't play Illarion anymore, Char became an 'honest' person etc.) there could be announced that applications are accepted again because there 'positions' free. Of course everyone else can play a thief as well – they just don't have the skill.

Anyway, it's just a suggestion. Please don't beat me too hard.
I just think that would give the 'normal' players the possibility to get to something 'special' for once as long as the RP is fitting. That would also work for other specifications, like paladins, priests or novices, etc.