Depends on your point of view, but the way marketing & sales is setup, anything electronic or computerized - from laptops to digital SLRs or even something like hearing aids - is slated for a 3 to 4 year lifespan.
Now physically you can keep using your digital SLR as long as you want, as long as it doesn't break down. I still have an 8 year old 1.3 MP digital camera (non SLR) that works fine - you just cannot find memory cards for it anymore.
After the 3-4 year period, you will find marketing kicks in and planned obsolence takes over - on any product. For example, you may find that batteries are either no longer available or are "special order' and will take 6 weeks to get in. In these cases, I then you go to 3rd party vendors like Batteries America and get the damned thing sent to you overnight.
So that's the real issue, IMO. I think some of the top of the line digital SLRs should last as long as my film SLRs (such as my FM2), but I've had so many experiences with trying to get anything 4-5 years old repaired or replaced, that's the real issue.
It's not just cameras. I have a friend who has a perfectly good telescope (8" SCT) from a major manufactuer. A single motor drive burned out. The company told him those motors are no longer made, buy a new telescope. My dad's 6 year old hearing aids, which work fine, needed an adjustment. The people who sold them say they don't have software for adjust them because they are more than 4 years old. Of course, he's never going to buy hearing aids from them again, but who's to say the next guy down the block isn't the same?
There's a very cynical part of me that sometimes thinks the whole "film is dead" mentality is nothing more than a grand conspiracy to brainwash us all into believing we all have to buy new cameras every three to four years, whereas in the past you could just keep using your same old camera for 20 years, and just go with new film. But hey, then I just go back on my meds (dark coffee), and everything is right with the world again.