There are two recent threads on the topic of what Matt Cutts and Google define as a relevant or related link. As many people know, Google has become very picky over the types of links you get and how much they matter to ones rankings. Also, recently, in Matt Cutts's blitz of interviews, he discussed links and Google's take on them. This sprung the recent discussion in the forums on relevant linking, in the eyes of Google.
Read those two threads to get the run down.