Before the Texas primary, I heard on NPR that national poles put it something like 55-41 Obama over McCain and 47-44 Hillary over McCain. If you were a Texas Republican, voting strategically, who would you vote for? And after leaving the Democratic Polling booth, would you really tell an exit poler what you had done? If you answered the exit pole taker honestly, it would ruin your strategy.   

Also, Obama lost a squeaker in the primary, but clobbered her in the caucus in Texas. I think a part of that was people who were willing to make a strategic vote in private but did not have the stones to do so in a public caucus.