So, I'm late to the game on this one (see -- I really am blocking out news of the dressage world), but I just recently became aware that the USDF changed their rules to allow posting trot in First Level. This is to take effect in the 2011 tests. I was taken aback by that decision, though in retrospect I don't know why that should have been the case. One can go on and on about how it's really for the good of the horse and his/her spine, rather than the dumbing down of dressage, but I find that very hard to believe. I get the arguments in favor of posting, really I do. It is better for the horse, in general, and I often post when warming my mare up for her reining sessions. However, I also feel that it's not ridiculous to ask a horse/rider combination to be able to sit the trot in a First Level test. You can practice a lot of posting at home, so as not to strain the horse's back, but in a show I don't see that it's a complete hardship to ask for sitting trot on the day of the test. If you've properly muscled up and prepared your horse, s/he should be able to handle the sitting, regardless of dressage education and/or age.
Really the larger problem here is that more and more people are completely unable to sit the trot properly. Even in my days as an instructor I found this to be the case more than I expected. And call be a bit old-school in my thinking, if you'd like, but I think that if you're prepared to show first level, you should be schooling second level at home. And if that's the case, then you'd better damn well be able to sit your trot.
Sitting trot is a very difficult skill to learn -- I get this, I truly do -- I spent months, years even, teaching certain students how to master the sitting trot. But just because it's difficult doesn't mean that it should be skipped over or shoved aside because judges are tired of seeing riders bouncing all over and jamming their mount's back. These riders should be properly penalized. Trainers should be prepared to explain to their students why they can't show first level until they master the sitting trot, no matter how long it takes. Call this USDF decision whatever you want, but it's clear to me that it's just a further example of the dumbing down of dressage. If the reiners that I ride with (not all of whom are riders with a great deal of finesse) can execute a relaxed, engaged sitting trot without a lot of thought or concentration, then our dressage riders should be able to do the same. Sometimes it's a matter of riders trying too hard and becoming rigid, sometimes it's a matter of riders being overmounted (how many 5'3" rider/17hh horse combinations do you see out there?), and sometimes it's just a matter of the rider having a weak core that could be aided by Pilates/Yoga work. Sitting trot is not an impossibility; I believe any rider can do it, otherwise there would be far less Western riders out there (and don't try to tell me it's because the jog is so slow that it makes it easy - sitting trot is sitting trot regardless of acceleration and cadence). We dressage riders just tend to overthink everything and/or overmount ourselves, which is outrageous in itself. If more riders opted to ride a smaller moving horse with a bit less impulsion, they'd be doing themselves more favors.
Funny story: just the other day my reining trainer exclaimed that no one had any business riding a horse bigger than 15.3hh, regardless of rider size. I don't totally agree with that statement, but dressage/hunter riders do tend to have a warped idea of how to match themselves with a horse. If you're a strong enough rider this doesn't matter as much, but if you're just a hobby enthusiast or just starting out in the sport, it can make a huge difference in how quickly you progress.
And if nothing else about this post sways you, then think of this: if you cannot sit your trot, then it doesn't just end there. As an instructor you quickly learn that sitting trot is just the most obvious expression of a larger problem with a rider's core lack of core strength and ability to engage the core, yet keep the hips and spine relaxed and flexible. The fact is if the rider is stiff in his/her sitting trot, you can bet that s/he is also stiff during halts, during transitions, half-halts and a myriad of other important movements essential to our sport. I've seen it proven time and time again. Removing sitting trot from first level, to my mind, just ensures that this problem will only become more endemic.