Sienna Miller : Sean Penn and Robin Wright Penn split up after (Sienna) Miller time??

Sienna Miller : Sean Penn and Robin Wright Penn split up after (Sienna) Miller time??

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."

Sienna Miller : Sean Penn and Robin Wright Penn split up after (Sienna) Miller time??