To me, the Lion Turtle's statement implies that the Avatar's existence was or is tied directly to the ability to bend: from his own phrasing, the Lion Turtle implies that no one could use bending other than spiritbending before the Avatar was created. As I said in the Sozin's Comet thread, the Lion Turtle, in his vast sum of what--three sentances?--has probably given us more information about the primordial history of the Avatar world than three seasons of knowledge spirits, moon-murdering admirals, and face-stealing centipedes.
I wouldn't be surprised if Spiritbending (shush you Energybenders! Until I see direct Word-of-God support for such an unwieldy term...) is and has been largely a nonpresence in the world since the Avatar's creation. The Guru is clearly doing something very funky that I'm brave enough to set forth as possible Spiritbending, even if he himself doesn't know the term. Likewise, the ignorance of the past Avatars may imply that Aang is the first--at least in a great while--to employ Spiritbending.
Hmmm, has anyone watched The Guru lately? Doesn't he claim to be from among a sibling group to the Air Nomads?
...On another, more audacious and pedantic note, does Spiritbending imply that a person's Chi is an entirely seperate notion? I mean, Waterbenders can direct a person's Chi--that's the basis of their healing power. A person's "Spirit" may then be a seperate concept, save that the only effect we've seen for Spiritbending is to strip one of their ability to bend...which is remarkably similar to what Ty Lee does by blocking the flow of Chi through the body. Could healing be a form of Spiritbending unknowingly practiced by the Water Tribes for uncounted ages? Healing is the only bending art we've seen that can indirectly manipulate an intangible such as chi.
The world may never know...rampant speculation man, away!
In all seriousness, we've probably got all the "straight" answers we're going to get about the Avatar world, if not forever than for a sufficiently long time. This is the point where the project has to be willing to analyze and extrapolate, and whether you choose to provide all collected analysis for the individual DMs to consider themselves or utilize variants or really go out on a limb and claim accuracy...well, I mean, it's a big step to consider.