A. One possible attack would be that this new TOS provision is unconscionable [1], and therefore unenforceable, to the extent it purported to give FB trademark rights beyond those provided by trademark law.
This argument might be supported by a longstanding public policy (at least in the U.S.): Trademark law gives certain rights to trademark "owners," not because they have some natural-right entitlement, but to protect the public's right not to be confused about the origin, sponsorship, or endorsement of the products and services they encounter in the marketplace [2]. A trademark owner can't enforce its trademark rights against another when there's no likelihood of confusion as to origin, sponsorship, or endorsement [3].
On the other hand, even if Facebook didn't prevail on a trademark infringement theory (requiring proof of a likelihood of confusion), it still might succeed in a lawsuit for dilution of a famous mark [4]. A court might view that as legitimizing the new FB TOS.
B. There might also be an antitrust challenge: By requiring users to agree not to use a term to which it (hypothetically) doesn't have trademark rights, Facebook might be guilty of an unlawful tie-out [5].
C. Finally, if FB doesn't require users to click on "I agree" again, and doesn't even notify them that the new TOS are in effect, then courts are unlikely to enforce the new TOS. The U.S. Court of Appeals for the Ninth Circuit already ruled to that effect in one case [6].
It'll be interesting to see what comes of this.
[1] http://en.wikipedia.org/wiki/Unconscionability
[2] http://en.wikipedia.org/wiki/Trademark#Public_policy
[3] http://cyber.law.harvard.edu/metaschool/fisher/domain/tm.htm...
[4] http://en.wikipedia.org/wiki/Trademark_dilution
[5] http://www.dechert.com/library/Analyzing%20IP%20License%20-%... at p. 7
[6] http://www.theiplawblog.com/archives/-cyberspace-law-ninth-c...