Warming up the muscles you are about to use is very important, more than you would think. It helps prevent injuries to the body part you are working out. But even more important you get rid of the waste that has been built up in your muscles and new fresh blood saturated in oxygen is now coming in.
Also, probably the most important part, you can train more intensely after you warm up. The muscles can contract harder and therefore you can get a better workout.
The most common and probably most effective way to warm up is by doing the same exercise with light weight and a lot of reps, around 12. Stretching should be a part of your warm up, stretch each muscle you are about to use for at least 8 seconds.

Source: http://weightlifting.cowebz.com/

I agree you should stretch, but this from The Stretching Roundtable:

http://www.t-nation.com/readTopic.do?id=459204

The current suggestion is to do static stretch before bed, dynamic stretching pre-WO and I would recommend foam rolling post-WO.

Agreed...also, one-two light weight warm-ups like that suggested in the article does nothing for motor unit preparedness to heavy weights. Warm-up, absoultely...but warm up the right way.

Ok, I didn't address the CNS warm-up. From Tony G, I modifed my warm-up:

1x5x60%
1x3x75%
1x3x90%

And that has done well for getting my CNS and joints ready for the heavy lift without fatiguing me.