While I'm not fluent in spanish, the gist I get from the article is that it was vetoed and sent back to Congress in 2006; perhaps it has only been passed recently?
You are absolutely right, in the first article it says the president vetoed the law the first time in 2006. Nevertheless the law was approved by congress in April of this year, so it appears like the NYTimes article is a few months late.
Well again, the news site saying the law was approved, is not from Mexico (TribunaLatina based in Spain). All law updates are published in the Diario Oficial de la Federación http://dof.gob.mx Searching that site for 'droga', or 'marihuana', 'posesión', etc. turnes out nothing relevant. Will have to call the family over there later to get more insight.
- http://www.eluniversal.com.mx/nacion/137975.html
Then it was finally approved on April of this year:
- http://www.tribunalatina.com/es/notices/mexico_despenaliza_l...
Edit: corrected description of links. Thanks PieSquared for noting the veto.