<p>I'm looking for a good server/client protocol supported in Python for making data requests/file transfers between one server and many clients.  Security is also an issue - so secure login would be a plus.  I've been looking into XML-RPC, but it looks to be a pretty old (and possibly unused these days?) protocol.</p>

Pyro
(Python Remote Objects) is fairly clever if all your server/clients are going to be in Python. I use
XMPP
alot though since I'm communicating with hosts that are not always Python. XMPP lends itself to being extended fairly easily too.
There is an excellent XMPP library for python called
PyXMPP
which is reasonably up to date and has no dependancy on Twisted.