Actually, there was a thread on it earlier this spring. You may want to poke around in the search function to find it.
What do you mean by "improved?" You mean the methodology, or the institutions themselves?
I think most people agree that most of the highly-ranked colleges have not undergone huge shifts in quality, rigor, or resources in the past two decades. But they have moved in rankings because the ranking methodology has changed considerably over time. Some people believe this means the rankings have improved, but there is still a lot of room for quibbling.