From what I can tell, there is a patent grant on the OSS version provided you pass the closed source compatibility suite.
> (iv) pass all test suites relating to the most recent published version of the specification of the Java 2 Platform, Standard Edition, that are available from SUN
That's an explicit Java patent grant separate from the open-sourcing (note the 2000 date, long predating OpenJDK). The exact patent status of OpenJDK is somewhat murky, though. Some people (incl. some lawyers) think that there's an implicit patent grant in the GPL, because the GPL explicitly prohibits distributing under the GPL any software that does not have all relevant patents licensed under royalty-free licenses. So if you gave someone a license to use your software under the GPL but didn't give them license to use a relevant patent, your licensing is self-contradictory: you will be simultaneously giving them a license to use your code under the GPL, and then that license is immediately terminated by the GPL's Section 7.
Therefore it's possible that a court might infer that giving someone a GPL license must imply a patent grant, because it would be absurd to assume that the licensor intended to give a license that was immediately self-terminating.
Courts do generally try to avoid interpretations of parties' actions that would assume they were meaning to undertake null or self-contradictory actions. But I'm not sure how much I'd bet on that outcome.
edit: Forgot another unlitigated possible angle. It's one thing to open-source something with a very explicit reservation of patent rights, like "this source code may be used under the terms of the [license], but Sun reserves any rights to patents that may cover portions of the software, which must be licensed separately". But what if you open-source it with a more blanket statement? With the OpenJDK release, there was a lot of rhetoric about openness from Sun, that amounted to: Java is now open source, you may use it for anything you want, as long as you comply with the terms of the GPL. It's possible that a "here, use this for free if you want" offer implies a license of any patents needed to use it. At the very least, it'd be somewhat dishonest to go up to someone and say, "hey, I've got this great thing you can use... here I'll give it to you free", and then come back five years later and accuse them of violating a patent. I mean, you're the one who gave it to them in the first place and told them it was okay to use it! So this might either imply a patent license, or at least present some sort of "inducement" or "estoppel" defense to a patent suit.
