The cruise industry has actually picked up to booking capacities similiar to before 9-11. Most ships are sailing at 90% capacity now if not fully booked due to spring break.
There were some fantastic deals last fall after 9-11. Both RCCL and Celebrity offered to waive the single suppliment rule for cruises booked by Dec 15th and travel before March 31st. I took advantage of this in December.
There are a few other specials which are periodically published here for other "single prices" for those who don't want to share a cabin with someone.