I don't have a problem per se with establishing a cap to deal with abuse.  My problem lies in two areas:

1.  The cap is the same whether you have 3Mbps service or 24 Mbps service.

2.  The cap is a fixed number, not a percentage of available bandwidth.

The fixed amount means that as speeds and and usage increase, the cap becomes more and more likely to come into play.  While most people wouldn't be affected by the cap now, what about when HD video becomes more widespread and service speeds top 100 Mbps?

Setting a fixed amount and not a percentage of purchased bandwdith is also grossly unfair to the person who has purchased wider bandwidth (and paid more for it).  Someone who purchased 3Mbps speed can utlize 25% of their available bandwidth (not unreasonable) with a 250GB cap whereas someone with 24Mbps (read: me) can only utlize a little more than 3% (which is a ridiculously low cap).  I've purchased 8 times the bandwidth.  If they are able to utilize 250 GB, I should be able to utilize 2TB.