Is it safe to ride a dual suspension bike in sub freezing temps?
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.