For many reasons, just dumping Comcast in favor of some DSL provider in your area would be a very smart move. All the cable companies have vastly oversold access to their networks, and are dragging their feet about upgrading capacity. A lot of teleco companies are even offering digital TV these days in bigger markets.
That all being said, the bandwidth cap is 250GB/mo for Comcast, so unless you're a heavy P2P user, you're not likely ever going to come close to that amount in a month. If you are a heavy P2P user, I hope you just have a penchant for downloading Linux distributions or other legal content, otherwise a bandwidth cap might be the least of your worries.
As for your actual question, I don't personally know of any such program, but talk to Comcast. It seems perfectly reasonable to me, that if they're going to impose this arbitrary bandwidth limit on customers, that they should provide some means for customers to monitor their consumption rates. Odds are they'll have something similar to what cell phone companies have, where you log in to a website, and it gives you an estimate of your usage as of an hour ago or a day ago, or something like that. You might also want to look for some European or Australian forums where bandwidth caps have been a way of life for many years. Odds are, if such a program exists, someone there will know about them. By and large Cnet is a North American oriented company, and our friends from across the pond don't find their way here all that often, so you'll need to go to them.