Yes: Cruises can be canceled. The most common reason is if a company charters the entire ship. Cruise lines make more money off a full ship charter than a regular sailing. It isn't common but it does happen. HAL is the cruise line I've seen the most of this type of thing happening.
If there is a mechanical problem they can cancel a cruise. The engine problems on Millennium class ships have resulted in a number of canceled sailings.
The good side of this is that cruise lines often give very nice compensation for cancellations. Many have offered full refunds and another free cruise.
Many times ship excursions are canceled because of too few people.
While cancellations do occur I would say that you have a 99.9% chance that your cruise will go as scheduled.