I have waited for this for so long. I'm glad someone finally took it up and and more importantly that its on a site as popular as dropbox. (this way hopefully the thinking will gain some traction)
Every time I'm forced to have a password with 3 or 4 character classes I sigh and think of that xkcd comic
Edit: also try typing the password from the xkcd comic here https://www.dropbox.com/register
nice touch