Last Tuesday's missing Crackdown demo will be available for download this Tuesday. Major Nelson says:

I don't usually like to talk about specific dates for content, but my sources indicate that it's ok this time : the long awaited 'Crackdown' demo will be available for Xbox Live Gold members next Tuesday (1/23) at 0200 PT.

Until then, check out 1UP's new feature "Five things to do in Crackdown besides killing people." To be fair, though, at least two of those five things might result in killing someone: #2 - shooting People, and #3 - Drive Recklessly. I guess it's not called Crackdown: On Violence! for a reason.