It depends on the camera.
The more expensive dSLR's like the Canon 1Ds Mark III and Nikon's D3 are rated to 300,000 shutter releases.
The Nikon D300 is rated to 150,000
Canon 40D & Nikon D80 are rated to 100,000
Nikon D40/40x & D50 are around 50,000
Canon XT & XTi and Nikon D70/D70s are around 50-75,000
Same for the Pentax, Olympus and Sony line. The Pentax K10D will have a longer life than the K100D.
Basically the more expensive the camera, the long the shutter will last. If you think about it, a pro photographer, in general, is going to take a lot more pictures than an amerture. A person looking for an entry level dSLR probably isn't going to be taking 10,000 pictures a year (I'm on the high side of an amerture and have taken about 6,500 in each of the last 2 years). A pro sports photographer may take a few thousand pictures for 1 event.
The shutter mechanisim can be replaced when they do fail. From what I've heard, its in the $200-500 range depending on the camera make/model.