My car battery appears to be leaking or has some type of residue coming from the terminals. There's basically white crusty powder on top of the battery. What could be causing this and how dangerous or potentially dangerous is this? Will I need a new battery? Is this type of thing covered under warranty?

I don't have a stereo system or anything drawing any additional power from the battery besides an alarm that was installed almost 2 years ago. My car is only 2 years old and I was pretty sure the Subaru battery should last longer than that.

It happens. It shouldn't collect very fast, though. If you get crusting in a matter of months your battery might be going out.

The errosion on the terminals can cause your connections to go bad. Get a toothbrush and some baking soda, take the terminals off, clean them, dry, reattach. This isn't a big deal at all and is good car maintenance anyway. You shouldn't really have to do this more often than every year.

You can also buy a battery terminal grease to minimize build up on the terminals

Thanks for the quick reply. I figured some build up was normal but it did seem like quite a bit had built up in about a months time. I'll clean it off and monitor the build up a little closer to see how quickly it comes back.

Does anyone know how the battery is covered under warranty?

The battery is covered under the 3/36 warranty although it may be pro-rated (not sure about that). Corrosion on the terminals is not cause for replacement, as long as the battery is still able to hold a charge and deliver its rated output.
If the alternator is overcharging the battery there may be some acid boiling over which can accelerate the corrosion. A voltage check should indicate if this is happening, with the engine running there should be 13.5-14.5 volts at the battery. 15 volts or higher is too much and will cause overcharging and boil-over.