According to the New York Times:

Bisphenol-A, or BPA, is used to make shatterproof plastic bottles, as a sealant in canned foods and in products ranging from CDs to sunglasses.In a draft risk assessment last August, the Food and Drug Administration said the levels of BPA to which children and adults are exposed do not pose a meaningful risk. Studies with laboratory animals have shown that it appears to accelerate puberty and pose a cancer risk. Most concerns about BPA focus on children, but some recent reports suggest it interferes with chemotherapy, and in adults it has been linked to higher risk of heart disease and diabetes.

Both of us have been trying to err on the side of caution, and the SIGG bottle was an investment we made prior to 2008, with the goal of avoiding plastic bottles. Unfortunately, SIGG, recently admitted that the plastic liners of their metal bottles contained BPA. (Source: Water Bottles Marketed To BPA-Fearing Parents Contained BPA All Along). Bottles manufactured after August 2008 contain no BPA whatsoever, however, when looking at the Sigg bottle we bought, it matched the cross-sectional view of the change in the Sigg bottle design as shown in the image below.

SIGG Cross Section View

Source: SIGG’s BPA Confession

According to Consumerist, SIGG appears to be doing the right thing by offering to replace older bottles made with the BPA liner with the new BPA-free EcoCare liner. For information on how to exchange your own bottle, send an e-mail to liners@mysigg.com or visit the SIGG Exchange Program.