Assuming a rollout of 2100 mm, which is probably in a ballpark of what you had, then 48.0 mph would correspond to 127.2 rpm, but this would only be if your pedaling was driving the bike at the speed, as opposed to coasting or "soft-pedaling" in which the bike is going faster than your "pedaling". Your actual rollout depends on the tire, inflation pressure, and weight on the bike as ridden, but is probably close enough that you'd be within a couple of rpm of 127.
42.0 mph in 53/12 would correspond to 121.5 rpm under the same assumptions.
Hmmm.. no way in hell I can do that rpm. This leads me to believe that I clearly get "going" at a certain speed, and let gravity take me to the final top speed. My guess here, of course, is that I was able to get to a higher speed before coasting at my comfort zone RPM than with the 12t.. There of course could be all sorts of subjectivity here too; like wind etc. Everything else bike wise is the same (except for the new Red 2011 shifters - but perhaps the internal cable routing seriously cut my drag!).
Thanks for the calculations though - I had a feeling I could count on you to provide them!!