The cancer thing is the problem with doing research on rats. The study cited above, rats were given the equivalent of a 900mgs dose of GW and it was done for the majority of the rats short lives.
This study further claimed GW-1516 increased carcinogenesis through a number of ways: increasing cell growth due to the inhibition of apoptosis (cell death), promoting cell proliferation (cell growth) by increasing cyclooxygenase-2 (COX2) and/or vascular endothelial growth factor (VEGF).
Since then studies have been done to see if this claim was correct and it was found that there were no changes in cell proliferation and failed to offer any evidence that PPARs such as GW increased cancer growth.
Lots of athletes have used this substance since it has been around and not one report of cancer through its use. remember again that humans are getting good results using 10mg at a much higher body weight that a rat who was given 900mg.
The Role of PPARs in Cancer