Ever since I got my early 2008 MacBook Pro, I have noticed electronic noises in the headphones whenever the Mac went silenct (for example, when a song would finish in iTunes). When I recently got Bose's over-the-ear headphones, I also started hearing static that I didn't notice before. I did some research and found this thread in the Apple Discussions.
The thread is currently 171 replies long, but the solution, found in the middle of the page, is to purchase a Volume Control Headphone Extension Cord from Radio Shack (their part number is 42-2559), or some other inline volume control. Plug in the inline volume control cable, then turn down the volume on the inline volume control and use the computer's volume to compensate. The key is that the control raises the impedance.
Note that this problem is entirely hardware related. Posters in the thread claim that this also affects late 2008 models as well. If you use headphones that already have an inline volume control, already have high impedance, or do not block outside sound very well, you might not have noticed this problem.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090113095435445