I have an SSH connection into work, and wanted to be able to access a Windows machine there via rdesktop.  I tried this:

$ ssh -L 3890:windows-machine:3890

$ rdesktop localhost:3890

But the rdesktop command hung for a long time then died.  Some poking around revealed this page: http://www.bluestream.org/Networking/SSHTunnelRDP.htm, which details an occasional problem that occurs when trying to connect to a forwarded port on localhost.  Their suggestion was to bind to a non-localhost interface.  Since their tutorial is for Putty, here’s the equivalent for OpenSSH:

$ ssh -L mymachinename:3890:windows-machine:3890

$ rdesktop mymachinename:3890

Where mymachinename is the actual hostname of your machine, which should in theory resolve to the IP of your ethernet adaptor.