I'm the first to say Comcast should come clean and declare actual guidelines, as it's only fair. But let's play a little math game for a second.
Comcast tells me I pay for 8 Mbps.
If I manage to saturate my download bandwidth for a whole month, I've downloaded 2.5 TB.
We gather that 250 GB (a tenth the maximum possible) is the threshold for getting a nastygram. Say you're downloading pr0n or TV shows or movies. And say it's high-def divx/xvid. An hour of high-def content with 5.1 channel surround sound is around 1 GB. So to get busted, you need to consistently download 250 hours of content per month, or almost 10 hours of downloaded TV per day.
Naturally when we start downloading 1080p content, the numbers will change, but for the time being this tells me I'm under the threshold, and it's probably safe to say that so is anyone else who isn't doing something quite extraordinary with their internet connection.