That was awesome. Harley comes off like a ditz but she can be damn clever when she really needs to.
I gotta wonder, though: what kind of therapy could possibly work on the Joker, given that they've presumably tried everything? (Short of a lobotomy, which I assume isn't legal in the DCU/DCAU or they would've done it by now.) Then again, the Joker seems to escape Arkham at will, so he probably doesn't stick around there nearly long enough for any meds, psychotherapy or ECT to take effect.