I have a Giant VT3 with dual suspension and I'm thinking of using it as a winter bike is there any special care I should take before heading out into the cold?
If you're riding in temperatures that are only a few degrees below zero, I wouldn't worry too much. As suggested elsewhere, make sure you keep a closer eye on maintenance (brushing off snow/ice, lubrication, etc).
The colder it gets though, the greater the risk that drivetrain components will fail. I've had at least free hubs, where the pawls would stop engaging (i.e. no forward propulsion). I'd say that all your parts will be more brittle in such conditions, so take it easier on your bike than you would normally.
Also, I've heard anecdotally that it's better to store your bike at sub-freezing temperatures if you ride regularly in the cold. The reasoning is that the snow and ice on your bike won't melt and find it's way into moving parts. This will be more important with the pivots on a dual suspension.
If you have air shocks, front or rear, be mindful of temperature impact on pressure. If you store your bike at about +20 C (room temperature) and ride it out into -10 to -20 C (seriously freezing weather), after a moment you will experience about 10% pressure drop in your shocks (and tires, too).
While 10% is not much, it can cause a noticeable change in shock's characteristics i.e. cause it to become softer. Of course, in winter that may not be such a bad thing...
If you're running oil in the forks then you might want to look at switching out for something that's meant for the colder temps but other than that I think it should be fine. The biggest thing I'd worry about is the salt/sand on the bike. Rinse it off after every ride with some water and store it where it can dry.
I used to ride every day straight through winter when I was in college. The problem with suspension bikes is that the suspension freezes up at cold temps. It doesn't even have to get below 0 for this to happen. You need to replace the lube with all synthetic lube if it doesn't have it already.
Years of winter riding in Vermont - never a problem with both oil and air suspension. Used Hayes's disc brakes which use the automotive brake fluid (DOT whatever). No problems, though I suspect the disc brakes which use mineral oil may have a problem.