So Malos has a solution: instead of tweaking the instances to us, how about tweaking us to the instances? He suggests a set of gear, much like the Heirloom gear, that matches your character to whatever instance you happen to step into -- if you enter Deadmines, it powers down your level 80 character to an appropriate power and level for the instance. That way, all Blizzard has to do is make one set of gear per class (that could even scale upwards, so they never have to make it again), and boom, every instance could be played at the standard difficulty by any character any time.
Will it happen? Probably not. But I really like the idea of tweaking the players, not the instances, and I think there's a lot of possibility there for Blizzard. They've had such a tough time trying to balance out content for all kinds of players (including all of the hard modes and extra gameplay in Ulduar), that it might be interesting to try and measure the difficulty by going the other way -- balancing players out for all kinds of content.