There is nothing in your computer that is perfect and hence it is true that the more you use it and the more you stress your computer, the greater the chance of a failure occurring.
However, the components in your computer should be stress-tested and should cope with this situation.
However, the biggest concern with running your cpu at 100% would be ensure correct heat dissipation as most cpu/gpu failures are caused by overheating.
Most computers need fans to get rid of the excessive heat, and sometimes something as simple as backing a computers fan's right up against a wall can interrupt the air flow.
Also fans are, in my experience, much more likely to fail than cpus, but if your cpu is maxed out and your fan has failed, or even not working properly, then it won't be long before your CPU will start overheating and if it continues running, it will then fail.
To help avoid that problem, most computers/bioses monitor the temperature of the CPU and shutdown the system it is gets too hot to avoid permanently damaging the cpu.
There are stress test programs such as this intel program which are designed to let you test your machine to make sure that it isn't overheating when under stress.
It should be obvious, but it is worth pointing out that running your computer at 100% all the time will increase your electricity bill (which may or may not be a concern depending on your financial circumstances and whether you are concerned about the environment).
On a personal note, I wouldn't think of leaving a computer maxed out just to display a 'nice' screensaver, but didn't have any problems running the Boinc cancer research 'screensaver'.