Amazon have been suffering a major outage for most of today which has taken down foursquare, reddit, and our favourite of all Cydia. Specifically the secure Cydia payment servers.
THIS MEANS ALL PAID CYDIA APPS WILL FAIL TO DOWNLOAD UNTIL THIS PROBLEM IS SOLVED.
If you are seeing 403 "forbidden" messages in Cydia. This is the reason. You will also not be able to get into the "My Account" section.
Because Cydia is a distributed package management solution, downloads from any of the other (free) repos should be unaffected unless they are also stored on Amazon infrastructure.
There is very little Saurik / Cydia can do about this. Nobody could or would expect a 12 hour outage at one of the largest infrastructure centres in the US.
You can check details of the outage at the North Virginia Data Centre at following URL:
AWS Service Health Dashboard - Apr 21, 2011