I can't get it to come up now but earlier this morning msn.com was sending all it's web search results through another redirect/tracking script. Both paid and free inktomi results were affected, the script they were using is at:
Could be that they want to see where the traffic is going. :) Since they have so many different PPC partners, it's kind of amazing they haven't previously wanted to track *everything* so that they had some kind of gauge for the revenue they were getting / losing from their various partners.
In my experience, PPC / unique click tracking will fall down occassionally, for a variety of reasons, so it only makes sense to add in their own redirect. I'm sure they have the bandwidth to monitor the requests being made.
Also, your suggestion of getting new urls to crawl is a pretty good one, and that would make sense, as well.
Welcome to WebmasterWorld, btw! Nice to have you here (excellent first post btw - great question.)