According to his biographer, it was the clever screen trick that convinced Jobs that Apple (AAPL) could make a successful cellphone.

According to Scott Forstall, it was a deal-breaker in the company’s negotiations with Samsung. “I don’t remember specifics,” the Apple senior vice president said in a pre-trial deposition. “I think it was just one of the things that Steve said, here’s something we invented. Don’t — don’t copy it. Don’t steal it.”

And it was one of five patents the Apple v. Samsung jury focused on in September when it hit the leading manufacturer of Google (GOOG) Android phones with $1.05 billion in damages.

Now, in what FOSS Patents‘ Florian Mueller calls a “non-final” ruling, the U.S. Patent and Trademark Office has declared the so-called ‘381 patent invalid.

Claim 19 of U.S. Patent No, 7,469,381 covered what Apple called “inertial scrolling,” but which came to be known during the trial as the “rubber band” effect. Here’s how Forstall described it in his deposition:

“Say you are scrolling something and you get to the end of it, as your finger descends down you pull it farther away from the edge and then when you let go, it bounces back.”

The USPTO has tentatively ruled that claim 19 is invalid because it was anticipated by two previously filed patents: A European one filed in 2003 by Luigi Lira and — ironically — a 2010 Apple patent filed by, among others, Bas Ording and Scott Forstall.

Ording, it turns out, was the Apple user interface designer who first showed Steve Jobs how to make an over-scrolled list bounce like a rubber band.

How the USPTO can rule an Apple patent invalid based on the company’s own prior art is one of those mysteries of the U.S. patent system. If I figure out how that works I’ll post an update.

Meanwhile, Samsung wasted no time filing a motion asking the court to throw out that part of its infringement judgement.