Very strange, I tried searching http://blog.pmarca.com for obama and romney and there were no results.

But I swear that back in 2007-2008 I remember him having blog posts about meeting Obama?

edit: found it at the archive http://pmarchive.com/an_hour_and_a_half_with_barack_obama

I guess it was just deleted over time as the blog.pmarca.com was made more professional.