Check this out:
I've always thought about this since the original trilogy (a lot), and here's my take. Obi-Wan is a master of stealth and invisibility, much like a ninja. Watch him closely in all 4 movies he's in. He's sneaking around the Death Star to disable the tractor beam in New Hope. In the new movies, he's also doing a lot of spying and avoiding detection, although he does manage to get captured (younger / less-honed skills). You can also couple this with his Jedi mind tricks; he's hidden in plain sight! "This isn't the Obi-Wan you're looking for!" Seriously, watch all of the movies, and tell me you don't pick up on this!
I believe he knew he had to blend in until Luke was of age. Tatooine? This had to be the last place Vader would want to go. Anakin saw his mother die after she was brutally beaten and raped by Sand People, and he also began his path to the dark side with murder, including women and children. It's safe to say that any of the Anakin left in Vader would never want to see that place again.
EDIT: yeah, counting his spirit, he was in all of the movies, not 4, inb4, etc. etc. ;)