My 2 cents...
IF you are in the market for a new 'stang anyway and aren't just looking to buy something with better mpg because of rising gas prices,
AND you have the money to buy a GT, don't opt for the V6 just because it gets 2 more mpg (EPA rating 19 mpg city V6 vs 17 mpg city for the GT).
Even at $4/gal you'd need to drive 100+ miles every day for a 2 or 3 mpg difference to make a substantial impact on your monthly budget. The $6k+ price difference between the cheapest V6 and cheapest GT can be a tougher hurdle for some people to manage, and if your working with a really tight budget (like me and 80% of other Americans) so that the rising gas prices are legitimately impacting your lifestyle, then the lower price of the V6 will have much more impact on your monthly expenses than the gas it uses.
As I said, this is only applicable IF you were already going to buy a new Mustang anyway. The higher payments of a new car will more than offset any gas savings, that is unless you trade a nice Mustang for a cheaper car like the Focus or Fiesta. (Even going that route, there are well optioned models of the Focus that still cost more than a base 'stang)
If it were me and I had the money to buy a GT, I would get the GT, I wouldn't opt for a V6 just for the mpg, you'll regret that decision the whole time you have the car, especially if a year or 2 down the road the turmoil in Libya settles down and gas prices come down with it.