I began to have doubts about the whole global warming *ahem* climate change thing since becoming exposed to the arguments on the other side, but what has tilted the scales firmly is the recent exposure of the internal CRU emails. No, not the contents of the emails themselves -- damaging as they are -- but the deafening silence from the mainstream media about this. Hopefully they're waiting for confirmation that the contents are not fake (which, by the way, is already available).
Update: Call me paranoid, but it looks like the Star Tribune story on this has been taken down. Google cache to the rescue. The doofuses forgot to take down the comments page for the story, however.