How cold are we talking? When it's colder than -10 (fahrenheit) all cars have trouble starting. Does it sound like the battery is having trouble finding the juice to start the car, or does it sound like the engine just won't get going?
As you said, you may just have a bad battery. One technique some friends of mine use when it's ridiculously frigid is to actually run the stereo, fans, and lights for a few minutes before starting. Their theory is that this runs enough current to get the battery warm, which then provides enough amperage to start the engine (turn everything off when you start it up, though). Disclaimer: I'm not sure if this technique actually accomplishes anything or if my friends are just wasting their time.
If it sounded like the engine just wasn't starting, and this has happened only once, it could be a fluke. I've heard that newer cars save the gas/air mixture settings from when they were last run. For example, say you you took your car out for a spin on thursday, when it was 35 degrees outside. You then let it sit until this morning, when it was 5 degrees. When you try to start it up, the computer is not providing enough gas for the colder, denser air.