Yeah, same here - on the one hand I realize that giving bonuses and modifiers for good speechifying is as silly as say giving another player attack bonuses because he can shoot ; but on the other hand speechifying is fun, so by all means let’s encourage it :).
I don’t give maluses though - well, unless you’re trying a really retarded lie for example, but that’s more about content than form. A lot of RPG players are not all that good at the social stuff, and you give them handicaps every time they open their mouths to say something “wrong” or are simply too shy to try and have a go, they’d never be able to play cha-based archetypes, smooth talkers, grifters and the like. Which sort of defeats the purpose of roleplaying. If they just want to roll for it then that’s fine by me ; and if they’re terrible at it, well, maybe their character can come up with something better 
I don’t really do negative modifiers except to curb stupid things like “let’s discuss stealing items in front of the shopkeep!” If somebody can’t make a riveting speech it’s not really a big deal, I just like to prevent the usual idiots from talking about raping the king’s daughter while they’re in front of the king that told them to rescue her.
I love when the most bizarre lies work out well, so rarely give penalties.
But I’ve had players with an absolute knack for saying Just The Wrong Thing in a situation, and frequently the rolls in those cases aren’t so much to see if the bluff succeeds, but to mitigate the consequences of the dumb.
Agreed, but there’s a difference between an outrageous lie and a stupid one. “Wa’n’t me whot shot up the saloon, sheriff, honest ! Was my twin brother. Mean as a rattler, that one, always sullyin’ mah good name !” is outrageous, and could work. It’s going to be a tough sell, but If it does, it’s good fun.
“I didn’t shoot nobody, I ain’t even armed !”, when the character is openly packing more heat than Duke Nukem ? Now that’s a stupid lie. And as you say, if there’s even a roll in that situation, it’s not so much to see if the sheriff is going to buy it but whether or not he’s going to feel personally insulted at being taken for an idiot ; or just laugh it off before hauling the PC away :).
I like what Warhammer FRP 3rd edition has done with Fellowship, its Charisma-equivalent: besides the obvious social stuff, it is the invocation skill stat for priests and you can use it in a few different ways in combat. Some Leadership support action cards that are fairly easy to use and give minor buffs, Insulting Blow that allows you to use it instead of Strength in melee and a couple Leadership attacks that can be very strong even if your own character is lousy in actual melee combat.
I tried playing a dour wizard with Fel 2 (3 is average, 4 is good, 5 excellent and 6 heroic) but just couldn’t get to like a character that wasn’t able use charm or guile at all. Playing a noble with Fel 4 now and enjoying it a lot more. I actually talked my way out of a conflict with a witch hunter. That would have turned very ugly if I had ended up fighting them.
I have a T-shirt that reads “Got Dex?” (A play on the old Got Milk? ad campaign).
In many systems, Dex (or Agility, or its equivalent) determines who acts first, makes you hit more often, and makes you harder to hit. In some, it also allows you to use better equipment (and in Diablo II, it also adds damage to a Bowazon’s arrows *and *determines shield blocking, which is a fixed percentage totally disregarding enemy accuracy and level). In other systems, it contributes to allowing you to act more times than your enemies (it adds to Speed in the Hero system, for example).
It’s relatively “harder,” more technical, and therefore somewhat resistant to the GM’s interpretation of social interaction like Int and Wisdom-type stats can be.
I have seen an entire campaign come to a fiery end on its opening night when someone failed a Dex roll.* For real.
Long before that, in my D&D days, I wrote an ode to Dexterity. I can no longer remember more than a stanza:
It’s good that Intelligence helps you to think,
But just wait 'til you hang from some black chasm’s brink –
Then you’ll wish that your fingers were nimble and quick
So your hands to the edge of the ledge just might stick.
*The GM’s wife, new to roleplaying games, slipped and fell into a pit of lava; when another player swung down on a rope in a desperate try to save her, the character anchoring the rope blew HIS dex roll, was yanked off the ledge himself, and all three of us plunged into lava and were killed. These were the first die rolls made in the campaign, and also the last, as one might imagine; we were never invited back.
Deadlands, on top of being a very Lovecraftian game, plays a lot with cowboy and western clichés ; and as such intimidation and cutting insults are very much part and parcel of the game system, and the combat system - insult a guy hard enough, or give him the old dead eyed stare straight in the eye a little too long for comfort, and he loses some of his actions for the round (because he loses his focus, that sort of thing).
Friend of mine devised a gimmick character to break these rules over his knee - a 14 year old kid who didn’t know how to shoot, nor how to fight, nor how to run or ride a horse, in fact he was pretty much useless at anything except taunting and insulting people. But he was so good at it that when he dissed you, you didn’t just lose *some *of your actions: you were almost guaranteed to lose them all. Paralyzed by sheer rage at this yapping little pipsqueak. It was gloriously stupid :p.
DM allowed it, because it was a one shot and the scenario was pretty light on combat anyway ; but the character proved invaluable in shutting down the annoying antics of the party’s dudebrah alpha dog gunslinger - he was the only one in the party who knew which part of a gun is the shooty end, and figured that gave him license to dictate orders and demean everyone else. Yeah, nope :).