I find it...odd that you say it "should" scale with level. What does it add to the game if AC is just another part of an arms race with BAB? If AC increases at a similar rate to attack rating, the higher attack rating doesn't really mean anything. You get a bigger bonus, but it's against a similarly bigger target number. If you're rolling +1 vs. AC 15 at level 1, +5 vs. AC 20 at level 5, and +10 vs. AC 25 at level 10, it means exactly the same thing: you need a 14 or higher to hit. If AC is kept at a much slower (but nevertheless real) progression, the bigger to-hit bonus means that you actually hit enemies more often. If you're rolling +1 vs. AC 5 at level 1, +5 vs. AC 2 at level 5, and +10 vs. AC -1 at level 10, you're hitting on a 14, 13, and 11 respectively: your actual average results increase with level. 3.x seems to take this aspect (which I think is really a good thing) out of the game, which is part of why it's not the game I run.