This "uncertainty" is a pretty persistent myth.
Yes, EVs are way cleaner (though of course exactly how much depends on the electricity source; but using average US electrical source, they are responsible for about half as much as a gas car - and using electricity gives both the utility and the owner a chance to improve it). The DOE, Sierra Club, Plug In America and Union of Concerned Scientists have gone over all the lifecycle studies and are very firmly behind their environmental benefits.
The problem is that there have been four "papers" (not peer-reviewed studies! they made many bone-headed mistakes; they have been dissected on these forums) that have been cited in blogs and editorials hundreds of times, as opposed to the several dozen good studies that you almost never see in the media. So if you just scan the media rather than read the studies (which of course is what almost everybody does), it's easy to think "gee, I guess they're not really sure". They are sure, there's just a calculated campaign to keep people confused. It has picked up (Plug In America follows media hits) since Tesla started getting accolades; that's because it's getting harder to convincingly say that electric cars are ugly, unsafe, no fun to drive and nobody wants one and it's impossible to make a profit selling them. So they have to concentrate on the most complex area that few people will bother to check out - lifecycle impact.
The really ironic thing is that so much attention is focused on this issue when, as Gear notes, it hardly matters - very few people base their car buying decisions on whether the cars are cleaner or not.