I've noticed that my MacBook laptop heats up about 20-30 degrees and my fan rpm increases quickly from 1800 rpm to 4600 rpm when I load Parallels with MS Vista Home Premium. For the short time I did the above test, the performance numbers above remained roughly the same. As soon as I closed Windows and Parallels, the system cooled back down and the fan rpm quickly spun down to 1800 rpm again. Is this normal? While I still intend to use Parallels for a few programs I have to run, it would seem to me that I am putting excess wear on the computer (especially the fan) by remaining in the Parallels mode for an extended period of time. The temperatures and fan speed were taken with iStatPro. Is there any setting I could change that might cut the above numbers down a bit. I assume the fan speed increase is strictly to take care of the higher system temperatures.
I wouldn't be surprised if the use of the system resources go up... You are running a second operating system, and you only have a 5400rpm drive in those MacBooks too, so those get crankin' as well. Take a look and see what your CPU usage is doing, both on the OS X side on the Windows side.
FWIW, I wouldn't worry about wearing out your fan, I would be more concerned about the processor (and it slowing down when it gets too hot). Get yourself SMCFanControl (free download, make a contribution) if you're worried about the temperature. It lets you beef up the fan speed manually on a variety of setings to cover spikes. Before I got it, my MB pro would sometimes shoot up to 80C or more before the fan caught up - not good for it at all.
