Hehe, well people having a cow about adults riding ponies couldn't ride at all then if they were in Iceland.
Over here the Icelandic ponies is the only horse there is (which is why they tend to be called horses since they're the only thing there is here and there isn't a word for a pony in the Icelandic language). It's been illegal for almost 1000 years to import horses into the country. That means that first of all no one is particularily worried about registering the horses that are bred since it's impossible for them to be anything other than purebred Icelandic horses, but also that if you want to ride something else, say a quarter horse, arab or a warmblood. You're out of luck. The sturdy, gaited ponies is what you've got to work with.
I've been riding since I was small (photographic proof, I'm the one on the black horse)
I'm 28 now and the first time I rode a non-Icelandic horse was a few months ago when I started taking lessons in Edinburgh and took the plunge since I couldn't get at any Icelandic horses
My dad rode on Aldinn several times for example, Aldinn is very easy going and tÃ¶lts easily with beginners. Baldur was a bit more trotty so you had to know what you were doing to get him to tÃ¶lt for you. It was a natural gait for him, just complete beginners wouldn't manage.
Here's a youtube clip of a guy riding very nice tÃ¶lt on an Icelandic mare:http://uk.youtube.com/watch?v=-7rWeWymJDw
Here's a world champion tÃ¶lter doing the range from slow to very very fast tÃ¶lt:http://uk.youtube.com/watch?v=g0Xw1RhuJDc
Baldur was not a competition horse, he could go very fast trotting (had to keep up in trail rides with the speed tÃ¶lting competition quality horses some of the people I went riding with had, although none of them were quite as fast as the one in the youtube clip) but his range on tÃ¶lt was more on the slow to medium speed of things. He also didn't have the high knee action. Which was just fine and I never put any weighted boots or anything like that on him either.
Anyway yeah, the Icelandic horses/ponies are really strong and built with thick bones and really powerful hindlegs etc. so if healthy and ridden correctly they don't have a problem carrying adults.