I have been having this problem for a couple days, emailing apple back and forth and last night it finally worked. Don't know what they did, but they sent this final email:
Dear Customer,
Thank you for your patience as we've investigated this issue. I sincerely apologize for the inconvenience that this has caused, as I know you've been waiting to download your purchase. I'm happy to report that this issue has been resolved, and you should be able to resume the download as normal at this time.
....
For those of you who are having problem my best advice is to keep on contacting Apple through email, and if you haven't started emailing them about this issue, go to
http://www.apple.com/support/itunes/store/download/