by Steve Ryfle In movies, romance novels, and even real life, it's called a relationship of convenience. It's a well-worn plot device, in which a man and woman hook up, perhaps even exchange wedding vows, for mutual gain rather than love. Maybe she wants a nice apartment, and he wants a work visa (see "Green Card"). Maybe he wants to run for president, and she wants ? to run for president (see Bill and Hillary).

In the entertainment world, the consensus is that relationships of convenience serve two main purposes. When two big stars are dating, it grabs headlines and helps promote their latest endeavors. When a gay actor wants to hide in the closet, it can help conceal his/her sexual orientation. And some such relationships, according to the rumor mill, are custom-made to accomplish both tasks.

Enter the newly minted Cruise-Holmes celebrity pact.

Even after marriages to Mimi Rogers and Nicole Kidman, even after a love affair with Penelope Cruz, some people refuse to believe Tom Cruise is a genuine ladies man. The wizened folks at People and Us magazines polled their readers recently, and found more than 60 percent believe the May-December fling (or, to be fair to Cruise, May-August) is a fake.

Perhaps it's the crass, overblown, smells-like-a-publicity-stunt way he and Katie Holmes have proclaimed their mutual affection on the eve of their twin summer blockbusters. That's an all-too-familiar story.

When Ashton Kutcher linked up with Brittany Murphy, the scribes said it was simply to promote their then-new film ("Just Married"), and when Kutcher traded Murphy in for Demi Moore, it was all to promote "Charlie's Angels."

When Ben Affleck and J. Lo were still news (yawn), it was all about promoting a couple of movies not worth mentioning now.

There's even speculation that the much-denied affair between "Mr. & Mrs. Smith" stars Brad Pitt and Angelina Jolie was hyped to help get their film to the top of the box office charts.

But the Cruise-Holmes relationship has hit a new level of intensity, even for Hollywood. With unconfirmed reports that Tom's so obsessed he gave Katie a GPS-equipped cell phone so he could know where she is at all times (which she supposedly gave back), fans are snapping up "Free Katie" t-shirts.

Why such a big push? Perhaps it's also the fact that Cruise (who once won a multimillion-dollar libel judgment from a gay porn actor, who alleged an affair with him) has had so much trouble shaking the perception that he's living an alternative lifestyle. Whatever the reason, gossip merchants are already proclaiming it one of the biggest relationships of convenience in Hollywood history.

It's a history dating back to the early days of the movie biz, when studios put morality clauses into actors' contracts and essentially forced gay stars deep into the closet. In 1930, MGM's head honcho Louis B. Mayer tried to force one of his biggest stars, William Haines, to end a love affair with a man and marry a woman instead; Haines refused and his acting career was destroyed, sending shockwaves through town. To protect their livelihoods and reputations, many stars entered into phony nuptials with a member of the opposite sex. These unions were known as "lavender marriages."

Through the decades, many high-profile stars have tied the knot under circumstances some deemed suspicious. Barbara Stanwyck was rumored to have had affairs with Marlene Dietrich and Joan Crawford when she married Robert Taylor, who was plagued by gay rumors of his own. Judy Garland's marriage to director Vincente Minnelli was rumored to be a cover for Minnelli's homosexuality. The marriage between Danny Kaye and composer Sylvia Fine was rumored to be purely professional. "Psycho" star Tony Perkins and Berry Berenson maintained the fa?ade of a happy family, complete with two children and a long-running marriage, despite the fact Perkins was gay. And perhaps the most famous case of all time is that of Rock Hudson, who succumbed to pressure from studio honchos and married his agent's secretary at the height of his stardom in 1955.

Even today, when many stars are openly gay, public homophobia is still a bugaboo for the celebrity trying to maintain an image. Thus, the rumors of sham star relationships continue: in recent years, the now-defunct marriages of Richard Gere and Cindy Crawford, Kirstie Alley and Parker Stevenson, Michael Jackson and his spouses (including Lisa Marie Presley) all provoked tabloid innuendo. In the early 1990s, Gere and Crawford became so incensed by stories implying their relationship wasn't genuine, and that it was on the rocks, they purchased-full page ads denying it.

The most oddball celebrity marriage in recent history, the 16-month-long union between Liza Minnelli and promoter David Gest, was forever dogged by talk of Gest's supposed homosexuality.

But back to Tom and Katie, to "War of the Worlds" and "Batman Begins."

Is it the real thing, or a master publicist's sly hand at work? Only time-and ticket sales-will tell.