Check out the trip reports and the "2012 Snow Level Thread" (the "water year" which started Oct. 1 is 2012) in the http://www.portlandhikers.org forums.
We are having unusually cold/dry weather for December. It's supposed to snow down to 2500 feet tonight, but only a few inches. Skiers are gnashing their teeth (and ruining their ski bottoms).
Wahtum Lake is at 4,000 feet and in a north-facing cirque, so you'll probably need snowshoes there. At lower elevations (3,000 feet and below), the trails are icy in places and you'll definitely need traction devices for your feet. The Eagle Creek trail is downright dangerous right now--lots of drips down the cliffs and of course they are all frozen on the trail! People are still trying to slither through without traction (a slow process), but on a cliffside trail it's risky business.
Be warned that conditions may change completely by late December. It could be anything from pouring rain ("Pineapple Express") to ice storms (in which case you're not going!) to more of what we're having now (freezing weather and strong east winds). Watch the trip reports on the above site just before you go, and keep close track of the weather forecasts.
Do note that around the winter solstice, we have almost 15 hours of darkness! That's especially true when it's cloudy (normal conditions) and in the deep, north-facing canyons of the Gorge, where you basically lose any "civil twilight." In other words, bring a book and extra headlamp batteries!