Actually, the Xenu story isn't told to a Scientologist until they reach the level known as Operating Thetan III, or OT III. Most Scientologists probably never know about that, and in fact those who do are not allowed to tell anyone who isn't a OT III.
Katie Holmes is loaded (with cash that is). She can buy her way up to OT III. This is what stops the layperson because it can cost from $20,000 to $300,000 or so (I have heard no definite number). I don't know how long it takes (and really don't want to research it), but if she doesn't believe in the aliens now, she will eventually. Rumor has it that Tom Cruise is OT VI or so (though some sites say that the OT naming is changed above OT III).