Hard drives are still affected by power cycling. I've been involved in the actual design of hard drives.... Granted, that was 3 years ago, but still fairly recent.
The issue is the spin-up of the drive, which means you have a power surge through the motor inductance, which means you just shortened the life of that drive.
Things are better than they used to be with hard drives -- most cheap drives are single platter, which means they don't have as much mass to spin. Manufacturers are also controlling the spin-up time to try to reduce the effects. Not all hard drives are built alike: some cheap Western Digitals or Maxtors come with a 90 warranty. The expensive WDs and Seagates come with a 5 year warranty.
Non-moving electronic parts typically don't have an issue with power cycling, but moving parts definitely do.
Side note: For those of you who choose to use the World Community Grid to allow those spare CPU cycles to be used, be aware that it is definitely shortening the life of your computer (every 10 degree C rise in temperature cuts the life in half), and it is also costing you dollars in power bills. A modern computer at idle (screen off) is only drawing about 30 watts. Get those numbers crunching on it, and that easily rises to 100 watts. Turn on the screensaver (graphics card CPU) and that can jump another 50 watts.