Lots of major travel sites default their searches to two guests. When I remember to switch it, the room price never seems to change. One can't very well eat two breakfast buffets, but for the occasional hotel that supplies a drink voucher I'm going to for a second one (after reading this thread).
Here's one plus for solo travelers... When you book Amex Fine Hotels & Resorts, the single traveler gets the full for-two meal credit, spa credit, etc. I've had a few awesome meals with that double allowance.