The primary rule has been that no living person is honored on a US stamp. Traditionally, they waited until someone had been dead for ten years before putting out a stamp, except for U.S. Presidents. There have been exceptions: Walt Disney's portrait was postified in 1968, two years after he died. Gemini astronaut Ed White's 1965 space walk was commemorated in 1967, a few months after he died in the Apollo 1 fire. And there've been several stamps showing the Apollo 11 astronauts on the moon, despite Neil Armstrong and Buzz Aldrin still being very much with us.
In some cases, the 10-year rule was more strictly enforced, leading to interesting omissions. In 1989, a block was issued honoring four of the five Best Picture nominees for 1939. They showed John Wayne from "Stagecoach," Gary Cooper from "Beau Geste," Judy Garland from "The Wizard of Oz," and Clark Gable and Vivien Leigh from "Gone With the Wind." All of these actors passed the "10 years gone" test, but the fifth nominee, "The Philadelphia Story," was not honored since their stars, Kathryn Hepburn and Jimmy Stewart, were still around. There's a lot more excruciating minutiae I could go into, but I'll just note that recently the official window was reduced to five years.
Now that whole rule seems to fly out the window in 2007 with the issuance of a sheet commemorating "Star Wars'" 30th anniversary, It's a good profit maker for the Postal Service after all: every stamp sold to collectors and not used to mail something is a 39¢ profit. But let's take a look: over a dozen characters from the six movies, most of them portrayed by actors who are very much among us.
I suppose we're seeing a new rule at work here. The stamp design is actually paintings of the characters, not photographs of the actors. That just might open the door to a whole new range of subjects for future issues.
Meantime, the post office is also taking votes on which of the ten stamps from the sheet will be issued later as an individual stamp. Absent one Jar Jar Binks on this sheet, we must throw our support to "old" Darth Vader. Emperor Palpatine will get his due with the passing of his acolyte, Darth W. Bush.