Seeing as modern vehicle mirrors have been shown to account for between 3 and 6% of the total drag on a car, I would think that deleting mirrors to hit CAFE standards would make sense. Not necessarily both mirrors, but even deleting one mirror would improve the aerodynamics of a vehicle (like the Civic VX and Metro XFi).
Plus, 3-6% is a pretty large gain. Adding variable valve timing is quoted as improving EPA fuel efficiency by 5%, and turbocharging is quoted as improving FE by 7.5% *. Redesigning an engine to add a turbo, or VVT, or direct injection is much more expensive than deleting mirrors and adding cameras in their place. Just my $0.02.
*FE statistics found at
Fuel Economy
Engine Technologies