I know this isn't the exact nuts and bolts of it, but seriously WTF?
Personally? I think they wanted a flashy story for the start of Detective Comics and it just fell short. BUT, I think it's going to all lead to a more Heath Ledger looking Joker somehow.
When it gets sewed back on for good, the mouth is gunna be too stretched from those strings pulling it into the smile shape, and he'll have to sew it somewhere and it'll end up looking like the Ledger Joker. Just my two cents.