One of the unspoken truths about the Star Wars cycle of movies is that Yoda and Obi-Wan Kenobi are jerks. Just reading those words seems like heresy, I know, but think about it: They lied to Luke Skywalker to manipulate him into considering Darth Vader an enemy to be beaten, rather than the former ally -- and Luke's father -- that he really was. Clearly, whoever taught the ethics class as the Jedi Academy needed a refresher course of their own.
Things may not have been as bad as it seemed from the final cuts of each movie, however. A deleted scene from Return of the Jedi has appeared online as part of an ongoing fan project to digitize the special features from an out-of-print laserdisc version of the movie, and in it, there's a hint that at least Obi-Wan knew the deception wasn't necessarily the best idea.
On his death bed, Yoda tells an offscreen Luke that "Obi-Wan would have told you [about the true fate of Anakin Skywalker] long ago, had I let him." It's a small detail, admittedly, but one that makes Kenobi a little bit less of a jerk, and perhaps even more sympathetic -- not to mention as changing the previously-harmonious relationship between Kenobi and Yoda slightly, as well.
Of course, the line didn't make the cut (literally), leaving this reading of the relationship entirely non-canonical… at least until one of the rumored Star Wars prequels decides to add this kind of disharmony back in. It's hardly out of the question.