Google STILL LIKES guest posting. Why? Time and time again, Google's head of webspam, Matt Cutts, said that Google prefers link sources where there is some EDITORIAL DECISION whether to create a link or not. Many pre-Penguin SEOs preferred sites that will automatically give you a backlink (example: article sites / blog farms). There is no trust established when you use an automatic link creation system. Guest posts need to be approved by the blog's owner so there is an editorial control system and Google thinks there's more trust here.
Before you go on a guest posting rampage, consider the following
Page rank of the source
Topic relevance of the source
Overall quality of the source
Traffic volume of the source
My final tip: Don't guest post just to get back links, guest post to expose and establish your BRAND.