This is a way to hide rate increases
I welcome the certainty of an openly disclosed download cap.
For the last few years I have dealt with the comcast bandwidth enforcers roughly once a year - I torrent legal audio and video recordings of live performances from bands that allow taping/trading. I finally got a second account and modem to avoid getting the annual threats, and given my current download patterns, I will actually be able to save money at 250G by dropping the second account.
One thing I have noticed is that the "secret" limit has been increasing slowly. I have been testing it occasionally, and can now do occasional months of 140G on one account without triggering the bandwidth police. A few years ago 100G would get you nailed in my area.
It appears that Comcast is increasing the download limit with an eye to the future... as more people start using internet TV and HD movie rentals, bandwidth demand will increase. A variable price structure is a first step toward tiered pricing based on usage. The punitive pricing for exceeding the allowed usage is similar to many cell-phone plans where you pick your limit (and price)and get screwed severely for every minute over. This leads to a situation where you usually buy a bigger plan than you think you'll need, just to be safe.