I had the exact same problem a while back. The battery light started to flash. As time went on, it would be on more often, and would go off if I revved the engine high or went on the highway pretty fast. I tossed the wal mart battery and put an optima yellow in. Solved the problem for a few hours, but then it was coming on again. I took it to the shop, they tested the alternator, and they couldn't get it to act up, but noticed very slight failure. They replaced it, problem solved.

Edit:

If you're worried about driving it, its safe. However, if your speedometer stops working, you better find a place to pull over within 15 minutes. Should be plenty of warning.