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It shouldn't have any negative impact at all. If you are concerned about it I recommend implementing Google's Font API instead, they're definitely not going to punish you for that! It will just show regular text if they don't have javascript enabled. Try it yourself, disable javascript in web developer firefox add-on to see what they see. I'd be primarily concerned about mobile visitors, in which case you should should disable cufon for mobile visitors.

Thanks for your response. Does it seem probable that this issue caused the 60% drop in SERP performance? The only other variable near the same time was changing hosting providers. We have moved to this provider for other clients, and never saw this kind of change, but that was usually at the beginning of the SEO campaign, not in the middle.