According to their website you'll get an email when you hit 80% utilization, and you can upgrade plans (or buy extra bandwidth for $5 / 20 GB) at any time. Also if you tell them in advance they can set your account to automatically buy more bandwidth rather than shutting off when you hit the limit.

Personally I've used about 20 GB in the past week, so I'm not worried about hitting my 500 GB cap.