Honestly I would think it would be more coverage than screen (in terms of massive battery drag)
The Amount of juice going to the radio can double in low saturation area's, i'm pretty sure making it the primary consumer of your watts!
Is there a mod (please tell me if you need to root, because i'm unfortunately stuck with an incredible that's not rootable yet until they update the reflasher. (came with 2.2) That tells you your signal strength in -db?