I'm concerned about that as well, I'm curious if if the non Butyl compound gasses at all. I'd be concerned that the inside of my car would smell like roofing tar, so I thought I'd ask to see if any fellow members had any experiences to share.
Something to consider: some of the new thinking in car audio/sound deadening is that you don't need to cover every square inch with the 'mat' material (aka dynamat, fatmat, etc). It's best quality is dampening sheet metal vibrations. These are most common in the middle of large panels of metal where it doesn't have as much support. The panels are sturdier at the edge and don't flex nearly as much, negating much of the value of the 'mat' in these areas. Closed-cell foam and mass-loaded vinyl are better at filtering out the remaining noises once you've stopped the metal from vibrating.
I'm not trying to say anyone who's done the full-coverage thing is wrong. In the past, I have done the 'full coverage' route with the mat. On my next install, however, I am going to try using limited butyl-based mat in the middle of any large panels (saving some $) and really pile on the closed-cell foam and mass-loaded vinyl product everywhere instead.
There's some decent reading on the subject at sounddeadenershowdown.com (I don't have any affiliation with this guy). Plenty more to be had at the diyma.com forums - probably enough to turn you blue in the face :-D