Global warming (if one believes in its effects) actually would/can cause cold weather in certain parts of the world, as well.
Warmer weather causes more precipitation; more snow. However, due to greater weather extremes and air movement as a result of temperature changes in some places, the location(s) of that precipitation is less predictable and/or consistent. Therefore we would likely see extended periods of drought in some areas and more snowfall/cold weather concentrate in other areas-sometimes in unexpected locations where the weather has been more moderate in the past.
The second thing that I heard is that as the ice cap melts, the changes in sea temperature will affect and slow ocean currents, which will eventually bring lower air temperatures across much of Europe. As the current slows (and stops?) the temperatures there actually cool and winters could become quite severe where they have been relatively mild in the past.
I got this off of the internet but it's an awesome answer. Plus, didn't anyone see The Day After Tomorrow? LOL It turned into an ice age.
However, I just saw what a smartass I looked like and yeah, I'd love to have some GW right now as well. It's freezing here too. 