There's currently a lively debate on License-review about OSI certification of CC0. Contention is largely focused on these patent issues. I dig the public domain, but until the courts, legislature, and/or international treaty say otherwise I think these dedications are hazardous to those downstream.

Edit: In reply to (and without attribution) the following deleted message:

  I believe there was commentary relating to unlicense's 
  approach on the OSI's license-discuss list in December. 
  In particular, one legal expert pointed out that you 
  can't open by dedicating your work to the public domain 
  (the 1st line) and then proceed to issue a license for 
  use.

  The CC0 license was held up for an example of how to do a 
  dedication plus fall-back as crafted by someone who has a 
  background in law... but this license explicitly reserves 
  patent rights and is therefore a poor substitute software 
  license (IMHO, IANAL).

  I'd love to see these "public domain statements" also  
  start to include a patent abandonment and/or promise not 
  to patent clause if the full intent is to be completely 
  free.