I see this thread his died off the past few months, but I came across this issue the past month.
I have an htc rezound and an iphone 4s, I had recently switched to using my iphone 4s because I have decent 3g speeds in my area (around 1.2mbs down) which is good enough for my day to day mobile surfing/radio streaming.
I switched to my iphone due to work related things. The first few weeks were fine, no dropped connections I could stream radio and still check my email. Once I hit around 3 to 3.5gbs my connection dropped down to .15 to .25mbs, and once I would try to multitask my 3g would completely drop.
I am just glad I found this thread, because I know in the past verizon has always denied capping data speeds.