Decks, Patios, Porches and Docks - post anchors

I'm building a pergola/shade roof in the backyard that will eventually have vines growing all over it. The question is can I use post anchors to secure the structure to the ground or must I cement the posts directly into the ground?

The structure will be free standing, and there will only be beams at the tops of the posts. The sides of the structure will be totally open.(no diagonal supports or mid level supports for the legs of this thing.)

Also, If I do need to directly cement the posts into the ground, what measures should I take to be sure that they don't rot?

Thanks.

There's no code or law to follow on this one (that I know of), but I prefer posts in cement. Many people disagree, but I think that the lateral stability you gain with cement around the post outweighs the rotting wood problem. I like to build up the concrete around the post to engourage water to drain away from the wood and make sure that it doesn't end up sitting in standing water. If it's not too windy where you live it will probably be strong enough without angle bracing and only post anchors.
Matt