As of August 2009, Sigg has revealed that the secret ingredient in their top-secret formula was, in fact, BPA. SierraDescents' position on BPA in water bottles has (hopefully) been clear: since good alternatives exist, I see no reason to use containers where BPA is present, and no need to get into endless arguments about just how dangerous (or not) BPA actually is.
What I find despicable, however, is that Sigg marketed itself to consumers as one of those BPA-free alternatives during the height of the BPA scare, even while it knew its linings in fact contained BPA. Further, Sigg aggressively challenged anyone who suggested otherwise via carefully worded denials that, in hindsight, appear to have been deliberately orchestrated to mislead consumers.
Sigg, we now learn, released a BPA-free lining in their bottles sometime prior to 2008. So why wait until now to reveal that their previous liners contained BPA? A cynic would suggest that allowed Sigg time to sell off the tainted stock and avoid a product recall. See here for information on how to tell if your Sigg bottle uses the old or new lining, plus more commentary on Sigg's behavior.
I'll be honest with you—I'm fairly outraged by all this. It was clear that Sigg was withholding information about its lining if you looked carefully enough at its statements (indeed, the SierraDescents review on the Sigg bottle calls the bottle "safe(?)" with regard to BPA and other toxicity/leaching issues). Still, Sigg's campaign targeting consumers seeking BPA-free products and Sigg's threats of legal action against those who suggested their bottles contained BPA is galling.
There is some talk that Sigg will replace the old bottles with new ones free of charge if you contact them. As a natural skeptic, I would not be quick to assume that the new lining is 100% safe. And there is certainly little reason to believe anything Sigg says about product safety whenever it conflicts with their sales goals. I think I'll keep one of their old bottles as a reminder not to blindly trust what corporations say. And I think I'll stick with my good old HDPE Nalgene bottle in the backcountry.