Arg, this thread frustrates me.

I weight 115lbs, but I'm 5'7"; I run 71 gear inches, not 65, or whatever was recommended; and I've only biked for about a year now; and yet, I can seated skid, easily, and I learned it probably within a week. And I'm pretty damn sure I'm not special either. Speed helps, but it's not a huge factor either. Hopping your wheel is unnecessary.

Ok, it's all about form/technique/whatever. Try this out, I taught a friend how to skid in a day without getting his nuts to the stem by having him do this, so really try it.

1. Pedal a bit, and then ever now and then go limp and let the pedals push your legs around
2. Do the same thing, but this time, as the pedal come backwards and up, let it push your body off the saddle. Just relax and stiffen your back leg a bit to do this.
3. Do the same thing but try to completely straighten your back leg as the pedal comes backwards and up, and let it push your body off the saddle
4. Do the same thing as #3, but when the pedals start to approach the point where the cranks are parallel to the ground, resist with your front foot by pulling. You should definitely feel a point where there is a great resistance to pedaling, and that should be when the cranks and parallel with the ground.

#4 is the key to skidding + seated skids.