It is implicit. IANAL, but it is supposedly very weak (see e.g. Novell-Microsoft deal). E.g. the European Commission, after analysis, thinks it does not protect forks. For a good explanation, see:

http://tech.slashdot.org/comments.pl?sid=1760290&cid=333...

FSF has been urging people to use GPLv3 because it has an explicit patent grant (amongst other reasons).