This means that the spokes (which are the weakest link of the wheel) do at least twice as much work on a bike equipped with disk brakes. I haven't seen statistics for this, but I'd be willing to bet that spoke breakage is higher on bicycles equipped with disk brakes. I also wonder if there is a higher rate of failure of the seat stays (which absorb braking force), since the force applied by the disk brake is several times higher than a rim-caliper brake. (see illustration)
On a rim-brake setup, you can think of the entire wheel as the "disk". Braking force is less, but more importantly, the braking force is being applied near the rim, and the only loads that are carried by the spokes are for driving force (which are inevitable) and structural (keeping the wheel true and round).
As long as the wheel is in fact true, and you can properly keep adjusted your rim-caliper brakes (of whatever type), They are a physically and structurally superior solution. It does require that you keep your wheels true, but I'm sorry, if you have wobbly wheels, your riding quality is going to be pretty poor no matter what. I will grant that using the rim for braking means that the rim is eventually going to wear out and you'll have to replace the rim (rebuild the wheel), but we're talking tens of thousands of miles here.
Of course, bike manufacturers are loading up their offerings with disc brakes, even for bikes that will never see off-road use (and of course many so-called "mountain bikes" will always be on a paved trail.) Here's an offering from Raleigh, here's one from Trek, here's a Civia, and even REI has one. These are all touted as premium bikes in their categories. But bear in mind that the disk brakes, while superior in an automotive setting, don't convey that much superiority in a bike that won't be used on unpaved trails.
Give me old-school cantilevers any time. They require some hand strength, but they never fade, and boy are they reliable.
Update: The excellent "Cozy Beehive" blog has a report on a disc brake induced failure here.