Over on Breeyark last year, Greg MacKenzie presented an interesting premise: Since armor class is in the game solely to determine the difficulty to hit for monsters attacking characters, it seems like a design flaw to stick magic-users forever at AC 9.
Therefore, he offers a new magic item called the Ring of Armor Class which allows magic-users to have a lower AC based on the chart below.
Here is what I think. It's a tantalizing idea, but fraught with error itself. Magic-Users have the advantage already of wearing no armor. They can outrun most anyone else, and carry more treasure. Additionally, they have fighters and clerics to protect them, if things are done properly.
Even if you decide that magic-users shouldn't be stuck on AC 9 forever, there are temporary solutions. Magic-users also have (duh) spells to protect them, if that's what they want. Stoneskin, Shield, Protection from Normal Missiles, the various Wall spells. But defense isn't the forte of magic-users. Offense is.
And the rings don't encumber, which is a huge advantage over normal armor.
Rings are only good for Magic-Users and for no other class. Maybe Thieves if they're clever. You can adjust the prices to suit your campaign prices of course. What do you think?