There seems to be an unwritten rule about this discussion. People who post thatarmor is underpowered have overpowered characters, while people that think it is fine have underpowered characters.
To which I would like to recommend to people who are using methods more favorable to higher stast then the book proscribes, then by all means use your own determination for AC values. Manual customisation. If your average character is well above a 25 point buy in stats then of course several things are going to seem underpowered and "not worth it". However if it is changed then it will become overpowered to the people who play games with more by-the-book generated stats.
I will not argue that unarmored is better than armored (spellcasting defenses are better, but they take a moment or two) but i will argue light armor prof is better then medium and heavy. I have noticed a definitive shying away from heavier armors if they have any sort of choice. (i got a 14 DEX? that sounds like its chain shirt time!) its like a walking death trap. you cant sneak, cant escape, and all for the price of having a point or two of armor higher then the guy escaping from the losing battle) breastplate is a joke, and THAT THE BEST MEDIUM ARMOR, one point of ac for slower movement and no mobility, no thanks.
I seriously hope the movement penalty gets addressed for the Fighter class AT LEAST. (at least higher level Paladins can have a horse to lug their clanky backsides into battle)
A Fighter with Mithral Full Plate has the following by 15th level:
+12 AC (+8 normal, +4 Armor Training)
+7 Dex allowed (+1 normal, +2 Mithral, +4 Armor Training)
0 Armor Check Penalty (-6 normal, +1 Masterwork, +3 Mithral, +4 Armor Training)
So a Fighter can have a Dex of 24 without losing any benefit while wearing his Mithral fullplate. That's enough for an 18 starting Dex, and a +6 enhancement from a belt of physical perfection. And that's an unlikely event for a character not devoted to Dex in the first place.
He also has 21 feats (22 if human). It wouldn't be hard to pick up the Fleet feat (see Jason's new feats announcement). Get it twice and you have the same speed as you had without armor.
Alternatively, you can buy boots of springing and striding for 5,500gp. Or, if you don't need the Acrobatics bonus, just make a custom magic item for the +10 speed at just 2,000gp. Hell, you can make one that doesn't even take a slot for 4,000gp.
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Overall, I don't think armor is hurting that much.
If the GM allows the magic item creation rules, then boots of expeditious retreat (+30 ft enhancement bonus to base speed) are a steal for 4,000gp. You can wear medium/heavy armor and still have a speed of 40 ft.Generally, you shouldn't use the "by spell" costs for making items that provide persistent bonuses. Down that path lies the robe of mage armor giving a +4 armor bonus to AC for 2,000 gp instead of the 16,000 gp you'd get by paying for the actual bonus, or the ring of protection from evil giving you a +2 deflection bonus to AC against 75% of opponents plus protection from mind control and summoned monsters for 4,000 gp.
You could try to reverse-engineer the cost from the boots of striding and springing. The BOSS give you two bonuses: +10 to speed, and +5 to Jump, all for 5,500 gp. +5 to Jump costs 2,500 gp (just like any other +5 skill item), leaving 3,000 gp for the +10 speed. Since the item has two bonuses, the lower cost gets multiplied by 1.5, and dividing by 1.5 gives us 2,000 gp for a "straight" +10 speed item.
There is no entry for "Speed bonus" on the magical item creation table, but there are entries for armor/AC (deflection, enhancement, or other). Ignoring that for a continuous spell effect item that grants an AC bonus is a complete twisting of the rules (a command word, x times per day or charged version, where there's a duration per use, is a valid use of a spell effect that grants AC). Basically, only if the effect cannot be created by the "Effect" entries do you go to the "Spell Effect" entries; you don't have armor of magic vestment (CL 4xN), you have +N armor.
The robe of mage armor would be 16,000gp and the ring of protection from evil would be: +2 AC/deflection (8,000gp) plus +2 Saves/resistance (4,000gp; additional effect on a slot item multiplies value by 1.5 for 6,000gp) plus additional effects from the spell (protection from possession and mental control, warding against physical contact from summoned creatures unless the summoned creature is Good or attacked; call it 4,000gp for both; additional effect on a slot item multiplies value by 1.5 for 6,000gp) = 8,000gp + 6,000gp + 6,000gp = 20,000gp (a bit more expensive than a ring of protection +3).
Reverse engineering boots of striding and springing is easy, since it follows the magic item creation rules exactly: + 5 skill bonus (5x5x100gp=2,500gp) plus continuous longstrider (1x1x2,000gp=2,000gp; spell duration of 1 hr/level gives no multiplier; second effect on a slot item multiplies value by 1.5 for 3,000gp) = 2,500gp + 3,000gp = 5,500gp.
For boots of expeditious retreat, the math is as follows: continous expeditious retreat (1x1x2,000gp=2,000gp; spell duration of 1 min/level multiplies value by 2 for 4,000gp) = 4,000gp.