I can't say definitively where it came from, but I knew about it for as long as I can remember. I used to act out Han rescuing Chewie from slavers with my Kenner action figures when I was a kid, (the slavers consisted of a couple of Jawas, a Tusken Raider, a Rodian, Snaggletooth, three Stormtroopers, a Deathstar Droid, and Walrusman, and of course the whole thing was masterminded by none other than Darth Vader with his insanely short telescoping lightsaber that was built right into his arm. Damn, I miss those days.) So it is definitely Pre-90's EU. Since my only non-movie exposure of SW as a kid was the comic books, I am betting that I got it from them.

I think the 'noble savage' is what GL was aiming for with Chewie. I think he was suppose to be to Han what Tonto was to the Lone Ranger. If you are old enough to remember The Lone Ranger, you might remember that Tonto followed The Lone Ranger around after he saved his life. I don't think there was a life debt in Tonto's case, it is simple what he chose to do. Tonto was always depicted as intelligent and an equal to the Lone Ranger, and that is always sort of the relationship I have seen between Han and Chewie.

I think I do recall some EU books talking about how even Chewie's descendants are obligated to protect Han with their lives (or maybe it was that Chewie was obligated to protect Han's decendants). That is a pretty crappy setup. Doesn't really make Han much better than the slavers he is suppose to have recued him from.

I also never liked the Chewbacca family situation that originated from the Holiday Special, where Chewie leaves his family to go follow Han around the galaxy and only comes to visit once a year or so. This makes him out to be a pretty lousy husband and father, and I always prefered to see him as a rather upright and honorable kind of guy (well, other than the whole smuggler/criminal thing).