What is a good rear wheel for a heavy load? I'm a heavy man, and I find I'm breaking spokes on my rear wheel fairly often.  Is there a particular wheel I can buy or anything I can do to prevent this?

Edit: I was having the problem with a hybrid with 700c wheels, but I gave up and sold that bike in preparation for my move to LA.  I'll be looking to purchase a new bike to use for commuting once I get out there, but I'm open as to style.  I gather we'll have Surly dealers out there, so I'll definitely be checking those out.
I had the same issues riding on pre-built 26" mountain bike wheels.  I'm 6'5" and weigh ~400lbs, so I break stuff left and right (including frame welds).  I have found two things that broke my wheel-breaking streak.

The first thing I found was the Surly Pugsley and the Endomorph (or Larry) tires that fit on it.  The 26"x4" tires are massive, and absorb any blow without threat to the wheels.  At the same time, I also went with wheels based around Large Marge rims, which have never needed to be trued (even through the frame break!).  

The other thing I found was beefy, hand built, wheels.  My second bike is a Surly Karate Monkey (a 29er) which I had wheels built for using 36 hole hubs (again a Surly product).  This wheel shows a bit more wear and tear (after 5 months of riding), but it hasn't broken anything.  I also run higher volume (but nowhere near the Endomorph) tires on this bike, with a pair of Schwalbe Big Apples.

Bottom line:  If your current equipment can't work for you, go big.  It will cost you a lot up front, but save a ton in the long run.

(I swear I don't work for Surly, though my bikes make it sound like I do.)