Likely 2012 Republican presidential candidate Mitt Romney appeared on CBS's "Late Show with David Letterman" Tuesday as part of his tour to promote his new book, "No Apology: The Case for American Greatness."Asked about potential rival Sarah Palin, Romney called her "terrific" and added, "she's got energy, passion." He then urged Letterman to "be careful what you say about her by the way," a reference to the feud between the two kicked off by Letterman's comments about Palin's daughter. Letterman grinned and nodded, saying, "I've had my…," before trailing off as the audience began laughing. Responded Romney: "She has a...