At what speed do professional cyclists/Giro Di Italia/TDF riders top out at? I haven't ridden in a while, but I don't remember ever passing 40mph. Even at that speed, pedaling was useless for me. At my lowest gear ratio I'd spin around 115-120rpm and I don't remember being able to catch up to propell the bike any faster. I've read reports of riders doing 50-55mph in the TDF regularly (perhaps a good deal faster?). At that speed are they just traveling on momentum or are they able to keep propelling themselves?

I did a little research earlier. The new Madone 6.9's larger chainring is 53t, the smallest casset sprocket is 12t (on the stock setup). Now, my Ultegra combination allows for 52:11t as opposed to the Madone's 53:11t. I shouldn't assume that is the bike of choice of the Trek sponsored riders, nor should I assume they use the stock cassette option but with that information our ratio's are quite similar.

Are they spinning at a cadence of 120+ rpm on hills, coasting down the steeper hill but carrying into it more momentum than I am, what's going on here?

It's probably a combination of both