Hmmmm? Have you?

No, but if I recall from my book learning, you don't need as much as you do for bottles...

Less head space in the keg = less sugar, is about all i know about it.
I bet its covered in how to brew, somewhere.
not sure where i read about it before.

I have. For me I use about the same amount of corn sugar. Figured I could always release some pressure if need be. It's been awhile. Worked well. I did it because my regulators were all full and I wanted to get a jump on carbing other kegs.

Whenever I dry hop in the keg, I carb with priming sugar (figure it helps eat some O2 from the hops). Also sometimes when the fridge is full I naturally carbonate. I try to shoot for a little under carbonated, hoping to get to the right level very quickly when I put the keg in the fridge.

Another potential reason for needing less priming sugar is that more carbonation is lost when bottling (since you have multiple transfers). Thus you have more CO2 already in solution when racking straight to the keg. I believe I heard that from Brew Strong.

I haven't found this to be the case, though. By using only slightly less than my software tells me to, I get about the right amount of carbonation after a week. The same software worked for bottles which just uses the standard carbonation tables you can find in How to Brew or elsewhere (~3/4 cup dextrose fro 2.5 volumes in 5 gallons). I might be getting the right carbonation in the keg because I only wait a week or two (as opposed to letting bottles age for 3+ weeks) - if one waited longer one might get overcarbonated beers?

One tip, though. Some kegs won't hold a seal below about 5psi, so if you try and naturally carbonate, put ~5psi into the keg before letting it sit. Otherwise all the CO2 created will just push itself around the seals.

Not with sugar, but I have carbed in the keg with brettanomyces.

+1 to this! The only time I've tried it I ended up with 3/4 of my beer on the floor because the poppet on the beverage out side wasn't quite sealed.

I've been too bitter to try it again since then...

I guess ... Not to discount your experience, but I've always thought the unsterilized priming sugar was just one more potential source of bacteria that could spoil my beer. I'd rather pop the CO2 off another keg, crank it up to 20 lbs or so, shake til it starts to slow down, then return the CO2 to whatever I'm serving and leave the new keg that way for a while and adjust as necessary when I'm ready to tap. This has worked pretty well for me.

The Rev