In a galaxy where the population is probably in the quadrillions, there are probably millions of people with the last name of Skywalker. One boy on Tatooine with the last name isn't gonna matter. Besides, given Anakin Skywalker's past, Obi-Wan knew that Vader would not only not give Tatooine any special attention, but would probably go out of his way to avoid Tatooine as its a reminder of his painful past...For whatever reason, for 19+ years, Luke Skywalker existed, and both the Emperor and Vader were completely oblivious to it. Keep in mind the sophistication of Obi-Wan's disguise...going from Obi-Wan Kenobi to Ben Kenobi...Honestly, if I were an imperial agent, I would assume a more elaborate disguise...if I came across the name Ben Kenobi in some census of tatooine or something, even if I had knowledge of Obi-Wan Kenobi, I would figure it can't be the same guy, as a Jedi Master surely would take greater steps to disguise himself. Truly, sometimes the greatest way to hide is to be out in the open.