You're still talking about a minimum of 12-19 servers though. I'd imagine that the average website doesn't use even that. I think that SO in particular is at 3 servers. Maybe if SO explodes, this will become an issue, but an optimization would likely have to make a difference of a few racks before it it's even worth considering making an optimization based soley on power concerns.
Whether an optimization is worth pursuing depends on the business requirements, how much the it costs to implement, and whether there are any additional gains beyond avoiding power, space, and personnel costs.
Simply picking an efficient (both for the developer, and in terms of implementation) language/platform can be enough to start with, and will help you avoid being boxed in later.
