This sounds daft, but the fact that feinting has no range limit moves it from "completely stupid" to "supoptimal, but could be fun".
You need Int 13 and Combat Expertise, which basically means you have to pay two feat slots for this since you'll almost never use CE otherwise. Ouch! But there will be some benefits down the line. You'll want high Bluff, which means you probably want to be a sorceror. (I'd say rakshasa bloodline, but it's not clear to me that their +5 on lies applies to feinting.) Alternately, an enchanter specialist works too. Consider dumping a third feat into Skill Focus (bluff).
So what do you do with this? I see two possibilities. One, you're a touch monkey, and you're using Improved Feint to reduce your enemies' touch AC. Pretty straightforward. All the standard cautions about touch attacks apply, especially the one about you being squishy. If you're going this route, you'd probably consider Aberrant Sorceror for its stretchy-arms 10' touch range.
Two, you're a blaster who uses a lot of ranged touch spells -- scorching ray, and the like. Again, you're using Feint to strip your enemies of their Dex bonus, making them more vulnerable to your attacks. For this build, you may want to consider Reach Spell.
Three, you're a multiclass caster/rogue who likes to sneak attack. This one could really shine. You can SA with a touch spell *or* a ranged touch out to 30' away (or 60' if you go the halfling sniper route). This actually looks like a plausible build for Arcane Tricksters and such.
The two-feat tax makes this a rather expensive tactic in build terms. However, your feints are going to succeed against a wide range of monsters more than half the time. Depending on the monster, this may do anything from "nothing" to "you just added +5 to your chance to hit."
Playing with some numbers:
Let's say you're a 4th level human sorceror throwing Scorching Ray. You have 12 Dex, 18 Cha. You've put max ranks in Bluff and also taken Skill Focus (Bluff), so you have +11. Let's put you up against some level-appropriate enemies!
Harpy (CR 4) -- DC 18 (10+BAB+Wis mod). You'll Feint successfully 70% of the time, stripping -3 off her AC.
Manticore (CR 5) -- DC 21 (effective +4 for being non-humanoid). Feint 55%, -1 AC
Wraith (CR 5) -- DC 15, Feint 85%, -3 AC
Shadow Mastiff (CR 5) -- DC 17, 75%, -2 AC
Djinni (CR 6) -- DC 19, 65%, -5 (!) AC
Skimming the Bestiary, it looks like you'll regularly be stripping 2 or 3 off the AC of your targets. In a few cases, like the Djinni, you'll peel more. (You cut the Djinni's AC from 14 to 9, which is pretty sweet.) This isn't amazing -- you spent two feats, and it's situational -- but it seems at least viable.
Let's note again that this is NOT an optimized build -- there are better ways to spend those two feats. But I think it's potentially a *viable* build; you can use it to build flavorful characters that don't actually suck. Or so ISTM.
Interesting ideas! Might not be as good as taking Weapon Focus (Ray) and Point Blank Shot but more versatile and quite flavorful!
Using it in concert with Combat Expertise means that you get a higher AC against the AoOs of casting and you compensate the penalty to hit of combat expertise with the AC reduction against your touch attacks! You even get a higher AC outside of your turn when making touch attacks.
[edit]: this kinda depends on your interpretation of combat expertise and whether or not you consider that using a touch attack for a spell means that you "declare that you are making an attack or a full-attack action with a melee weapon". Touch is considered an armed attack but I'm not sure the free action to touch as part of casting a spell counts towards declaring that you're making an attack.
Feint wrote:
If successful, the next melee attack you make against the target does not allow him to use his Dexterity bonus to AC (if any).
Upon further reading there is one thing you've got wrong. Feint only benefits melee attacks, so not ranged touch attacks.
Benefit: Whenever you use feint to cause an opponent to lose his Dexterity bonus, he loses that bonus until the beginning of your next turn, in addition to losing his Dexterity bonus against your next attack.
Greater feint might help you out with this. Personally i don't think that greater feint was intended to remove the melee limitation. But trickshot of the archer (fighter) archetype most certainly was. I don't know if that is of any use for casters though... maybe a gish?
No, greater feint doesn't help. And trickshots require you to use a bow... it's looking like the application is pretty limited here; basically aberrant sorcerors and (some) caster/rogue types.
