During the clone wars Chewbacca was instrumental in the defense of Kashyyyk, fighting along side Yoda.

20 years later Chewie again meets a jedi master.

When Obi Wan goes to Mos Eisley to find a ship to take him and Luke to Alderaan, Obi Wan walks straight over to Chewbacca, the same wookiee who befriended Yoda all those years before.

Coincidence?
I'm not so sure

If Chewbacca knew Yoda, then maybe he also knew Obi Wan too.

Its always struck me as odd that Obi Wan could walk into the canina and straight away find the first mate on a ship that suited them perfectly. It almost seemed like Ben knew that Chewie would be there. As if he knew exactly where to find a ship to take them.

Then, ROTS came along and suddenly it makes sense. Now we know that Chewbacca and Yoda were friends, its not hard to theorise that Chewie also knew Obi Wan. That, while on Tatooine, Obi Wan had perhaps visited Mos Eisley to meet with Chewie.

With this in mind, a few minor details in ANH seem to show themselves a little clearer. For instance, when Ben ignites his lightsaber, cutting the arm off of , Chewie doesn't even flinch, almost as if he'd seen Obi Wan use it before.
Then a few minutes later, when Obi Wan asks "if its a fast ship" and Han replies "you've never heard of the Millenium Falcon?" Chewbacca seems to grin, as though he knows that Obi Wan has heard of her before.

Yes, with all things considered it really does seem like Obi Wan and Chewbacca did know each other before hand. And its a good thing too, if Chewbacca hadn't have been there the galaxy would have had a very different fate!

If you haven't already, check out Chewbacca Week over at nob01's blog