An Anonymous Coward (as they call them) on Slashdot provides the clearest explanation I've seen so far.

I'm only quoting the original comment here, see the discussion on Slashdot for follow-ups:

The problem is that to be a compatible Java implementation you must pass the TCK. To get a hold of the TCK you must agree that your Java implementation has a limited field of use, namely desktop computers. That means you have to add a clause to your licence that tells your users where they can use the software - no such clause exists in any open source licence I'm aware of.

Sure you can use the OpenJDK, you can even fork it, but therein lies the problem... you can't, because if you do and you want to claim it's a compatible implementation you have to pass the TCK. So you have to licence the TCK, then you have to add a field of use restriction to your licence, but that's incompatible with the GPL that the OpenJDK GPL requires you to licence under.

End result, you can have Oracle Java or 'Open'JDK

The ASF don't have a political axe to grind with the GPL, aren't firing a salvo in some imaginary war based on their view of free; It's about a contractual obligation Oracle has to release the TCK to the ASF. An obligation Sun had and failed to meet and that Oracle continues to fail to meet.

The ASF was re-elected to the JCP with 95% of the vote. No other elected member had anywhere near that. The members spoke with their vote and consequently the ASF leaving the JCP would be big news in a war with Oracle, nobody else. The ASF is outside core Java and the work of the JCP probably the biggest single contributor to the Java ecosystem. Their threat to leave the JCP would seriously damage it and Oracle's commitment to opensource's credibility.

You can only have Oracle Java or 'Open'JDK - there's no way out until Oracle honors the agreement.

I have also started collecting a list of links about the whole thing, at delicious.com/bdelacretaz/oraclemess.