In a Mother Jones article, Practical Values: Paying for My Hot Air, Kimberly Lasagor explores the topic and comes to some interesting conclusions.
Still, there's a bigger issue here. The whole idea of atonement by credit card seems counterintuitive. It's as if we're saying polluting is okay, as long as you can afford to pay. But Strasdas says we should think of it as a last resort. Yes, we should all strive to emit less carbon, but some emissions are harder to avoid ("You cannot have planes that are flying on renewable energy, at least not in the foreseeable future," he points out). That's where offsets can help. "This is not the way out. This is a temporary relief of pressure on the earth's atmosphere," Strasdas says. "For the time being I think it's a very good way to bridge the gap."
The full article is worth a read. It brings out a number of points that you've probably thought about yourself.