Leaving a modern PC on 24/7 is actually better for it. The electricity they use is negligible if it is just sitting there doing "nothing".
With the monitor off, HD spun down and no CPU intensive operations going on you are probably drawing 10-20 watts. Which means it will cost about £7 more to run each year. You will save this on reduced wear and tear, provided you keep PCs long enough to break that is!
In terms of degredation of semi-conductors, this is not a new effect. This is why precision semi-conductors (such as op-amps in very sensitive lab equipment) have time-constants on their datasheets so you know to throw away the component when it reaches a certain age. Not really a big issue in normal digital equipment.
However I can see why it would occur with smaller manufacturing processes. 65 micron CPUs are to come out in 2005, so at the current rate of development we will probably hit sub-micron chips in a decade or so. Hopefully by then they will have solved the problem!
In terms of leaving a PC on, many people do, and have no problems. The only reason I don't with mine is that my main PC is in my bedroom and I can't stand the noise! If you just want something to surf and e-mail with, might I suggest a "mini-itx" computer. These are often totally silent as they have no fans, have miniscule power consumption and are cheap to buy. The motherboard comes with everything integrated into it, and enthusiasts build their own cases. Great fun!
Brilliant!