I have a 2005 Pontiac Grand Prix, twice, when it has been very cold out, the display said "charging system failure", it only said it for a few seconds, and the car seems to be running fine. SHould I be concerned?

Keep an eye on it. If the problem becomes more frequent you can have the battery and alternator tested for free at an auto parts store. I am guessing that moisture is freezing in the electrical brushes and interrupting the current for just a moment and back on again. If it doesn't go away in warmer weather or with a warmer engine, get it tested soon. It won't leave you stranded as a strong battery can power a car's electricals for up to two hours with a dead alternator. But be safe.

If your battery is more than 3 years old, I would just replace it (but go ahead and have it checked anyway). Cold weather can kill a weak battery quickly by not allowing a sufficient charge to be held for starting the vehicle. If your battery is newer, I would think about investing in a battery booster and keep it with you in the car. Good ones on the market these days that will hold a charge good enough to start your car for up to a year.