After an avalanche of bad news for Samsung in its legal brawl against Apple, the sun seems to be finally rising for the Android king. A key patent used by Apple to prove Sammy’s infringement and copying in several Android devices has been invalidated by the USPTO, making Samsung’s defense following the August California jury verdict a lot easier than expected.

The US Patent and Trademark Office’s ruling is tentative for the time being, meaning that it can and will be contested by Apple, but this is surely a (partial) win that Samsung needs to savor following a pretty long and painful string of defeats.

The iPhone makers have been granted $1 billion in damages after Samsung’s been found guilty of infringing a number of patents owned by the Cupertino-based company, but Apple was hoping to get that sum increased, as well as to ban sales on several devices from the competition. Sammy’s defense was detailed in a filing made last Friday, but, as we told you yesterday, this didn’t look very strong, being based on arguable software workarounds and unsubstantial evidence.

However, now that the rubber-banding patent (No. 7,469,381) has been deemed invalid, Samsung can start hoping for a decrease in damages, at the minimum. Judge Koh has already been informed about USPTO’s decision, and, as Florian Mueller from FOSS Patents notes, this could lead to an approval of Sammy’s Rule 50 (overrule-the-jury) motion.

It’s almost impossible though for the invalidation to lead to an overturning of the entire verdict, seeing as Apple still has a number of other software and design-related patents in play. But if the ‘381 patent can be rejected due to “lack of novelty” and existence of prior art to Cupertino’s supposedly “original” work and if the decision will stand, why couldn’t the ‘163 and ‘915 patents be themselves unauthenticated? After all, we know very well that Apple’s “innovations” are questionable.

What do you guys think today’s USPTO ruling means on the long haul? Can Samsung start a winning streak now and overturn the $1 billion California verdict? Or will Apple convince the US Patent Office to reconsider?