Once again, thanks all for your help. I'm glad I waited to ask before
doing anything.. I might have messed with the wrong fitting and caused
us all a lot of grief!

On one end of the finned-pipe "radiators" is the fitting that I
described.. Under all the crusted sediment, it looks like there is a
hex cap on top that I thought might be to bleed it out. On the *OTHER*
side, however, is something much like Paul described: It has a
square-drive stem on top, much like a commerical/institutional water
valve. I think THAT is where I should bleed it out (after I go to the
plumbing store and get a "key") And I'll remember to take the WD40
with me!

As for if the pump is really turning or not.. Well, I can't vouch for
there actually being water flow in the pipes, but the intense heat in
the main "trunk" indirectly suggests to me that the water is flowing
under pressure. The steep temperature gradient in the branch plumbing
makes me thing there's something else wrong other than overall lack of
system flow. The temperature goes from too-hot-to-touch to
room-temperature over 2 feet of pipe. The entire run of trunkline is
in the "too-hot-to-touch" range