  Well I went to see my dad off and then to see "The Day After Tomorrow". I will say that it is indeed a pretty damn good movie. We will see however a trotting out of "scientists" that will say the science is hokey. Well taken as a whole it may seem a little hokey until you break it down like I did.
First the shelf of ice that breaks off. There is currently a very large chunk of Antarctica that has broken off or is in the process of breaking off and will soon be the largest iceburg that has been recorded in modern times. It is huge. Once that much ice melts into the ocean it is sure to cause a slight cooling because it will drift and melt. Once it melts the ocean will cool maybe a degree or so and also change the salinity of the ocean. Again these changes will be slight in human terms. The ocean currents that I have spoken of before do indeed exist and we do have the knowledge of if you shift these currents you will cool the ocean and thus the planet also cooling the air temps. Here is the big if question. Scientists don't know how much ocean cooling or salinity shift will cause these currents to shift or stop all together. But we do know something will happen but not the extent. We know that changing paterns in the air flows and masses change our weather paterns. We know it because we can now see it pretty clearly. If the jetstream is changed drastically (or even slightly for all we know) we will see many problems.
Tornadoes in Hollywood, blizzards in New Delhi. We just don't know. But we do know that the Gulfstream (ocean current) is linked with the Jetstream somehow. Also we know for fact that melting ice raises water levels and we have seen the ocean slightly rise (and it is rising at ever increasing rates) over the past years. Ok so now comes the hokey science. Or the part that can be passed of as hokey but hopefully I can convince a few people that it is probable.
The Ice Age that starts in hours, days, or even a few months. We know it is possible. It has happened in the past. We have wooly mamouths that were frozen in place with food in their mouths and stomachs as if they were eating and then flash frozen. It is possible and it has already happened. It happened about ten thousand (10,000) years ago. A blink of a geological eye. This is where the movie kind of makes up some science...well sort of.
The make the case that the shifting and eventual stopping of the Gulfstream effects the Jetstream while the water level is rising. As the Jetstream changes huge storms start to form similar to hurricanes only they form over the northern land masses toward the north pole. I think that the storms would be effected by the season in some way as to where they will form but that is my opinion.
If the northern hemisphere is in fall winter and the southern is spring summer then the storms form north and vice versa. These huge storms reach the troposphere where the air is very very cold. Those storms happen now but they are small and the air that is sucked down as it is in a hurricane is warmed as it reaches the surface by water and air because it is over the water. Over land there is only air to warm it. A small storm would cause cooling of a few degrees. A large storm over land would suck down to much air at a time and allow it to not warm in the surrounding air. It would be a -150F blast of air to the ground. That would flash freeze almost anything and anyone. We know hurricanes do this and that is how they form and get strength along with warm water. Warm water, cold air and you get hurricanes. Well over land we don't know for sure and that is where the movie really speculates. However seeing that it has happened before it is plausible that it will happen again and we will be the cause of it.
It was cause before by a buildup of greenhouse gasses that took tens of thousands of years to do naturally through volcanoes and other things. This time we have volcanoes and the rampant burning of fossil fuels that only adds to the problem. Now we can dismiss it all as hokey science but it will only be hokey till it happens. I believe it will happen. As was made evident over and over again in the movie there is no timetable we can follow because we didn't really record it the last time it happened.
This time we will. You can slowly warm a frog in a bucket of water until he boils without him jumping out. When you toss a frog into boiling water he will try to jump out. I believe the water is already boiling but because we are all so used to these slight changes we will only see that the water is boiling around us when things actually freeze. Go watch the movie...it is impressive and entertaining all at once...and the science if you dig into it a bit isn't far off at all. Peace. 
