Really, really hoping that Crispin Glover isn't being considered. I think he does a great "silent and creepy guy" but it's like typecasting all over again. For me, this Joker really has to live up to what this film began. Scarecrow isn't a maniacal villain in every scene...the ability for Murphy to do both the cold and calculating Crane and the maniacal Scarecrow is what makes the character that much more interesting.
The last thing I want is someone who is either...
a) Unable to capture the maniacal aspect of the character
or
b) Unable to create the image of someone who became horribly disturbed, not just someone who "is" that way
I think Glover fits into the latter category...he's been nothing but creepy ever since Back to the Future. And even there he was a freakin' peeping tom. That's no Joker, damnit.
I think it is worth a look to allow Penn to screentest for it...I think that he could put together a strong character. It's whether or not he's the best option that is the question...I think there are probably better Jokers out there, but it's definitely something that they should look into further.
And, to be honest, I'm hoping Joker remains a periphery villain in the second film. I think he would be a great little distraction for Batman; someone terrorizing Batman while he attempts to battle a larger threat. Have one fantastic showdown between them, introducing the character, and then hold off on something major until a third film.