The exact reduction in field strength for a cell phone with increasing distance from living tissue to antenna may not be a perfect square law but that is probably a fair approximation (closer than linear). With that in mind, considering the antenna is typically pressed to the users temple, I'd think the potential for harmful effects should not be so ephemeral as to not give cause for concern to a person otherwise concerned with the effect of the EM fields in the passenger compartment of a Prius.
I am not claiming that the efects of cellphone usage are a clear and present danger, certain to cause brain tumors or even increase the probability significantly (there are reports from folks much better informed than I). I'm stating that for someone to worry about EM in the Prius passenger compartment due to "leakage" past the shielding (metal dash, hood, and other body components but to not worry about a radio transmitter pressed to their temple or hours of exposure to a CRT, is inconsistent and strangely selective.
FWIW there have been studies and there are published findings. In the Government sector, I have read a published report about an inch or so thick in 8 1/2 by 11 size that covered what is known and what is suspected regarding EM exposure in humans. Unfortunately, that report predated widespread distribution of Prii. If I recall correctly the prelim version hit the streets about 1996 give or take a few years. It does cover cell phones and other hand held radio transmitters. Being a ham radio operator and also having a federal liscense for working on commercial radio telephones and ships radar systems, I had a vested interest in this topic predating Prii.
Does "swallow a camel but strain at a gnat" sound familiar?
ADENDUM: And now, this just in...
http://www.popsci.com/popsci/medicine/a ... 49,00.html
Popular science isn't exactly the "National Enquirer." It might be prudent to consider a headset to get the antenna off the user's temple and a phone with an antenna connection to allow an external antenna for car usage. The external antenna would give much better performance while mobile and would be a good thing even if there were no long term health considerations.
There are "glove" type adaptors that you slide your cell phone into to couple it to an external antenna to be used with phones that have no provision for external antenas. They are not nearly as good as a direct connect external antenna but are sill better than the antenna on the phone. Small simple mag mounts will allow changing the antenna from vehicle to vehicle if you don't want to install a permanent antenna in each vehicle.
Of course the Toyota Prius and BlueTooth technology can reduce your exposure via cell phone to head proximity.
I'm not spouting DOOM AND GLOOM, just suggesting prudence.

Pat

(Prudent Pat!)