[plt-scheme] fluid-let vs. parameterize

What's the difference between fluid-let and parameterize (aside from
the syntax)?  Does fluid-let deal with threads the same way?  If not,
is there a situation where you'd want to to use fluid-let instead of
parameterize?  I'm currently using fluid-let for dynamic scoping, but
I'm thinking parameterize might be better, and I can't really tell
from the MzScheme manual what the practical advantages would be.

--dougo at place.org