Ha!
Ha! My first post on this forum. I was about to write how amazing this is, that I was just thinking about this same thing for my campaign, until I realized that the thread starter (Felnar) is one of the people in my group! Hahaha! Well, I thought it was funny...
Anyway, I could just call Felnar up on the phone, but I guess posting my ideas here would be better, because then others could critique them.
Here is what I am thinking. It does modify the rules a bit, but I'm coming from 2nd edition, so really it just makes it more along those lines anyway:
Instead of having a "max-dex bonus" for armor, there would be a "dexterity penalty". This would reduce your effective dexterity by said number for anything related to agility (AC, saving throws, certain skills; but not missle attacks, weapon finess, etc). If this ends up giving you a dex penalty, then you do NOT take that penalty to your AC. (Dex penalties to AC don't make too much sense anyways, to me. So if you are flat-footed and have dex penalties, you get better AC???)
I'm not sure about the exact dex penalties yet, but I'm thinking something like -1 for studded leather, up to -3 or -4 for plate. I'd also throw in similar penalties for shields, such as -1 for small, -2 for large and -3 for tower, or possibly -0/-1/-2 respectively. Since these dex penalties would double-up on the armor check penalties to some degree, I might also reduce that penalty by 1 for every -2 to dex the armor gives.
Encumberance would have to do the same thing: give a dex penalty instead of a max dex bonus.
The reason behind this? Well, it doesn't make much sense to me that someone with 12 dex wearing full plate would dodge blows just as well as someone with 18 dex (or 25 or 40). Game-mechanics-wise, it then makes all armors just a sliding scale: either low AC bonus, but low dex penalty, or high AC bonus, and high dex penalty. Deciding which armor to choose is just a matter of deciding how much dex you are willing to sacrifice.
With this system, an 'armor specialization' feat could then decrease the dex-penalty for that type of armor, for one thing. I also like the 2nd ed rule where it halved the encumberence of the armor, although with the movement restrictions, it kind of doesn't make a difference, since you are still restricted. Of course, doing it the other way around (reducing armor move penalties) also doesn't make a difference, since the weight of the armor is almost always going to bump a character into medium encumberance.
Just my $0.02. Let me know what you guys think.