In the most recent issue of Popular Photography & Imaging magazine, the had a feature on doing wildlife photography. One of their recommendations is to get a 500mm lens with optical image stabilization. You don't even want to know how much that costs . . . but if you want to play the guessing game, you might want to start at $5,000.
But that's not the point I want to get at here. Optical image stabilization, a technology apparently introduced by Canon, is the point. I believe some earlier technologies were digital, so you can imagine how bad they were. This one works differently.
A camera's lens is made up of several pieces of glass, called elements. They might look something like this sloppy diagram I made in MSPaint (I was basing this diagram on the elements of a microscope, so the picture isn't entirely accurate, but you get the idea). The light travels through all of the elements before it hits the film or digital sensor.
Normally these elements don't move. But with optical image stabilization, they are allowed to float a little bit in such a way that it doesn't matter if the front elements shake a little bit.
At first I put the idea out of my mind, thinking that it couldn't make much of a difference. But Canon claims that it makes a two stop difference. That is, if, with your focal length, you could shoot without shake at 1/500 of a second without image stabilization, you could shoot without shake at 1/125 of a second with it. This means, the way I figure it, that you could potentially handhold a 500mm lens under some fairly weak lighting conditions, especially considering that such a lens probably doesn't have a very wide aperture (many are limited to about f5).
So if you're looking into buying a new lens, especially a longer one, I would definitely consider looking into Canon's new technology.