Last week, a Vanity Fair spokeswoman acknowledged that the May cover shot by Annie Leibovitz <A HREF="http://www.americanphotomag.com/article.asp?section_id=3&article_id=3500">was "inspired" by "Ballet Society,"</A> a 1948 portrait by Irving Penn. The revelation confirmed what one source has been whispering in Pop Photo's ears for years. Leibovitz is by any account one of her generation's greatest photographers, but as the evidence of her "artful borrowing" piles up, it's hard not to see a little chink in her armor.