A good base layer is essential for mountain biking to wick sweat away from your body - sweating on cold weather rides can cool you down too much. The Armourfit range of base layers (made by rugby specialists Canterbury, but good for MTBing) are available in Hot and Cold versions - Hot to keep you cool and Cold to keep you warm.
The Cold is made from lycra/polyester/nylon mix and it has an AB100 anti-bacterial finish to stop you ponging after a day in the saddle. In fact, we used ours for five rides without washing it and it still didn't smell.
The fit is skin tight - this is comfy (no jogger's nipple), and according to Canterbury, it helps to prevent muscle oscillation, which leads to muscle fatigue. We're not sure how helpful this is for MTBing but this is still a great base layer for all outdoor activities.