Google’s AdWords blog, Inside Adwords, has posted an advance notice of new changes to their display URL policy. According to the post, redirects, vanity URLs, and certain uses of tracking URLs will no longer be acceptable. This new policy will go into effect on April 1st and will be strictly enforced. The blog post gives a brief example of what will still be allowed in regards to tracking URLs and sub-domains.
I don’t think most advertisers will be unduly affected by this new policy, but I encourage you to read up on the new policy so you aren’t caught off guard.