The moment of truth in Sarah Polley’s "Take This Waltz" occurs when two of its principals are buck naked. Showering in the locker room after a dopey swimming class, Margot (Michelle Williams), a married woman secretly tempted to stray, and her sister-in-law Geraldine (Sarah Silverman), more happily married but a recovering alcoholic, pontificate on the inevitability of domestic boredom. “New things become old,”