I did this on my very first extract batch because I got impatient. As far as flavor and all that goes, I don't believe it effects the beer at all. In fact, IIRC the extensive boil time isn't for anything but the hops - otherwise we could just throw cold ingredients into a carboy and pitch yeast, much like apfelwein. But the hops provide different qualities depending on when they're added. The only difference I believe you'll see with a pre-boil addition is a darker beer when compared side-by-side with a late-extract addition.