Just some random speculation, but I seemed to be catching the implication that attempting to human transmutation was the reason Ed and his teacher can do alchemy without the circle. It kind of makes sence, with the equivilent trade thing. It takes something from you, and earlier someone had mentioned how they got nothing back, (in exchange for the arm leg and body) but I wonder if maybe that ability was the gain.
There's a couple of years in between when that happened and the first time Ed did do it without the circle, the reason for that gap could be more to the fact that he never considered not using the circle before. But that again isn't completely solid logic, as he would have seen his teacher do it before, so the idea should have been there. Maybe you need a certain level of skill/power to go with having done Human Transmutation.
Then again, why can't Al do it? Possiably because he was more of a helper in the transmutation rather than the instigator/creator... maybe the skill thing...
hmmmm... is a puzzlement ^_^
edit: Here's a link to picture of the tatoos. Which adds a whole new devolpment... No real spoilers in the pics.