Wow... that's a freaking theft... The wallet was online I assume?
Electrum - dropbox was compromised (with plain txt for the pwd, I know I know..)
Sorry for the loss. No lecture about security practices from me, I'm sure you're already crushed about this enough.
That said:
Electrum password alone isn't sufficient (assuming the attacker got it through your dropbox plain text file), he'd also need a copy of the electrum wallet file. Any idea how that was accessed?
Or did you actually mean the 12-word "master seed"?