Tadd, definitely an interesting idea. I'm playing with ideas for alternate gas tanks as well, so I got on Google and here is a summary of the first three results.
1. One website (Chemical Resistance Chart - Jensen Inert Products
) describes PVC's gasoline resistance as "good".
2. Another website shows where a guy did a 54-hour gasoline immersion test and decided the same thing (SpudTech Archive • View topic - PVC Pipe Gasoline Resistance Test
3. Here is a Wiki question (Can PVC pipe be used to handle gasoline and will the regular glue used hold? - Yahoo! Answers
), I don't have any idea what the qualifications of these people are, but they all seem to be in agreement.
I think the most definitive way to determine the gas resistance of a PVC tank would be to go for it - build it, keep an eye on it to ensure its safety, and then us how it went