You might find this article of interest (I tried to find the source materials, but was unable to ... the URLs in the article point to a discussion of their VDSL network, not their pricing plans).
This is one of the items that gets to the heart of the "network neutrality" discussion ... should ISPs be able to offer tiered pricing? There is already a tradeoff between price and quality ... if you want more bandwidth, you pay more. If you want better quality, you can build an MPLS network (assuming you know the endpoints).
If you extend the analogy to non-telecom services, you can purchase mail with different delivery times at different rates ... so why should you be able to do this for internet services as well?