When downloading updates, most people stick with the default server set in software sources. The problem is best seen when new point releases come out. On download day, the main server that most people use gets bogged down and download speeds drop to almost nothing, consequently causing the upgrade to take a day or two. Other people have proposed setting apt to use bit torrent/P2P to balance the load among other Ubuntu users. However, that could pose problems, such as ISP P2P restrictions, ISP bandwidth caps, company/university policy, etc. Ubuntu already contains part of the answer to this problem. In "software sources", you have the option to "select best server". Ubuntu then scans for the mirror providing the best bandwidth. Why couldn't Ubuntu automatically do this and select the best three mirrors, balancing the downloads between all of them? This would have a similar affect to P2P but just be using the mirrors, thereby avoiding the problems that come with P2P.
Since posting this, someone has kindly pointed out that my understanding of the technicalities of all this is flawed. It is just an idea meant to spur thinking in this area in hopes of finding a possible solution. Please post better solutions if you have them.