While the token answer right now is "No, cellphones and low SARs levels can't cause cancer", it isn't exactly a settled subject. I'm not saying cellphones do cause cancer, but over the last 10 years or so, more and more "studies"( I put studies in quotations because if you study anything long enough you can find what you want to find) have come out suggesting there might be a link. Most of the time, the link is a slight % increase among people who are heavy users for many years. Because more and more people are drifting towards texting, FaceTime, and other forms of communication other than calling we never see a concrete link between cellphones and cancer as the way we use them are ever changing.
With that being said, if there is a link, bluetooth usage might be what exposes it. With just about every major phone manufacture now doing away with the headphone jack, we are being forced to adopted bluetooth headphone technology. While the SAR is much lower in many cases, the length of time used is, by design, much longer. Headphones are meant to be used for long hours, therefore if there is some sort of cumulative response it will be bluetooth headphones that force it rather than cellphones.