There's never a simple reason for a couple to part after 20 years. But if Sienna Miller didn't cause Sean Penn and Robin Wright Penn to split up, some claim she may not have been a good influence.

A source recalls a party in a suite at a New York hotel where they both were staying.

"Sienna was sitting on Sean's lap," according the source. "She was dressed very sexily. She had her arm around his neck." That night, claims the source, they stayed up quite late.

One Penn friend maintains there was never anything romantic between them. "Sienna is like that with everyone," says the friend. "She's very physical. She drapes herself over people she likes. She doesn't mean a lot by it."

Miller's rep insists that suggestions that the actress, now dating Rhys Ifans, was ever more than friends with Penn are "outrageously untrue. Sienna is a very good friend of Robin. She adores and respects Robin."

Still, you wonder if the up-till-dawn scene might trouble a wife left home with two kids.

Speaking of breakups, Penn has filed for a journalistic divorce from San Francisco Chronicle editor Phil Bronstein.

Penn, who has contributed dispatches to the Chronicle, sent a letter to Bronstein on Tuesday in response to a satirical piece about celebrities who've met Venezuelan President Hugo Chavez.

"I could only hope this great city was more clever than its increasingly lamebrain paper," wrote Penn, who was angry that Chavez had been called a dictator. "Chavez was democratically elected … The Chronicle should take comfort that The New York Times generally does no better than them here.

"Yet, in the paper's attack on Oliver Stone, who, agree with him or not, has at least the balls, passion and intellectual curiosity to pursue information away from a porcelain dumping bowl and a desperate newspaper, we see another attempt to marginalize the outspoken.

"I have a reasonably good sense of humor … but I felt so embarrassed, even for the Chronicle. … Its Datebook editorials have become Mad Magazine for small-minded cowards and former writers of substance. Who could that be, Phil?"

Penn signed the letter "former SF Chronicle contributing reporter."

Bronstein, who's survived a split with wife Sharon Stone, called Penn "a great actor and a great director. People get riled up about a lot of things, particularly in this day and age; they get to express themselves. We were more than happy to print his letter."


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NO wonder Kate Hudson divorced rocker Chris Robinson - he doesn't bathe. The Black Crowes frontman fumes about the music scene in Relix magazine: "Everything is squeaky clean. Even the punk-rock bands are clean . . . You're not supposed to get your hair done that much, you know what I mean? You're supposed to look like [bleep] because you did it yourself and you never bathe. You can't be punk rock if you buy all your [bleep] at Fred Segal."


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ATTENTION, Dita Von Teese: You have a package coming from Jeremy Piven. The "Entourage" star picked out two black satin, sequined mini-dresses from the Be Seduced line for a woman he told a swag-suite clerk he had a crush on. But as he browsed at Kari Feinstein's suite at Social Hollywood in LA last week, he was forced to come clean about the identity of his "brunette and curvy" lady friend because he didn't know her size. He finally told the clerk the dresses were for Von Teese, the burlesque-dancing ex-wife of Marilyn Manson. "He said he likes her, but it's not official yet," the employee told Page Six. Piven's rep said the duo are "not dating. He's busy shooting a movie." But apparently he's not too busy to flirt. Von Teese's lawyer, Keith Fink, told us Piven has seen her performance and he shyly said hello to her when he ran into her at an LA newsstand right before he bought the dresses. "She is single, and she's not dating, so he should step up to the plate because there's a lot of guys interested," said Fink.

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TIFFANY Fallon, who looks great in boots and body paint on the cover of this month's Playboy, is steamed about the way she was booted off "Celebrity Apprentice." Calling Donald Trump's show "a shark tank," the brunette tells Steppin' Out's Chaunce Hayden: "I didn't even get to say goodbye to anybody . . . I was tossed out right away. I left the boardroom and was shuffled into an elevator and kicked out the door. I was really p - - - ed off! I thought, 'My time is not over!' But you can't ask the driver to march your a - - back upstairs."