“Although Samsung has a right to compete, it does not have a right to compete unfairly, by flooding the market with infringing products,” Judge Lucy Koh wrote. “As a patent holder, Apple has a valid right to exclude others from practicing Apple’s invention.”

The ruling won’t do immediate harm to Samsung, which filed an appeal five hours later. Stores that carry the device can continue to sell what inventory they still have, and the order does not affect the Galaxy Tab 8.9 or the  Galaxy Tab 10.1N, an updated version designed to get around a similar ban in Germany.

Nonetheless, it’s an important ruling — symbolically and legally — and the timing is significant. Samsung is the only maker of tablets based on Google’s (GOOG) Android operating system to have made any headway against Apple in one of the fastest-growing markets for electronic devices, and the court found that it did so with a tablet “virtually indistinguishable” from the iPad.

Moreover, the injuction was issued just as the next generation of iPad competitors enters the market. Microsoft (MSFT) unveiled its family of tablets — dubbed Surface — last week,  and Google is expected to introduce one of its own later today.

Ironically, the judge who handed Apple its first major victory in the U.S. for a design patent — as distinguished from a utility patent (see Apple’s secret weapon) — denied the “obvious validity” of the very same patent six months earlier.

Below: A timeline of Apple’s zig-zag path to victory.

April 2010: Apple introduces the original iPad in San Francisco
Feb. 2011: Samsung unveils the Galaxy Tab 10.1 in Barcelona
March 2011: Apple unveils the thinner iPad 2. Samsung’s CEO vows to redesign his tablet
May 2011: Samsung gives away 5,000 Galaxy Tab 10.1s to attendees at Google I/O
June 2011: Apple claims Samsung infringed D’889, a design patent signed by Steve Jobs
July 2011: Apple requests a preliminary injunction blocking the sale and importation of the Galaxy Tab 10.1
Dec. 2011: Judge Koh denies Apple’s request based on Samsung’s argument that their tablet could have been derived from two tablet designs that preceded the iPad, and not from D’889
May 2012: A Federal appeals court overrules Koh, writing that the two prior tablet designs were too different from the iPad to be the basis for Samsung’s tablet
June 2012: In a high-profile ruling, Judge Richard Posner denies a similar injunction request in Apple v. Motorola
June 2012: Judge Koh grants Apple its preliminary injunction against Samsung
July 2012: The trial is scheduled to begin July 30. If Apple prevails, Samsung could be liable for substantial damages

You can read Judge Koh’s ruling here. The appeals court ruling that changed her mind is here.