Today is the first day I've heard of chemtrails. Was browsing on Craigslist and found posts about the "chemtrail conspiracy" and didn't feel like reading through tons of BS to find out what the "conspiracy" was. So I found it on wikipedia instead.
Now, the idea of the government spraying chemicals isn't something I would dismiss immediately as I hate the government and think they hide tons and tons of crap from everyone. However, the idea of all the governments working together on this? Doesn't sound too likely to me. But you never know. And if they are trying to fix and repair the o-zone or whatever, then they probably wouldn't be trying to hide that.