Apple - Can I get S.M.A.R.T. support on external hard drives?

Yes (maybe).

There's an open-source kernel driver OS-X-SAT-SMART-Driver for Mac OS X that will work for some USB and Firewire enclosures. It uses SAT (SCSI ATA Translation) to pass the SMART commands through to the hard drive, so only works if your enclosure supports SAT.

SAT isn't something enclosures seem to advertise support for, but the one I use (WD MyBook Studio Edition) happens to support it, so it works for me. It'll probably be quicker to test the software with your drive/enclosure, than to find out if SAT is supported in the specs.