Logging into dotmac and trying to view the iDisk brings up another browser window with a log-in menu. There is a statement saying "password will be sent in the clear". Does this mean people can steal my password to my iDisk? Is it safe to do this?

Well, yes they can - if you are doing it over an insecure wireless connection, or they have somehow tapped into the wire. Tapping into the wire is *very* unlikely (unless you do this at work, in which case the Admins may monitor traffic, but they will probably not be looking for you - they're looking for a specific problem). Over an unencrypted wireless connection - that's much more likely. So I would avoid doing this at a free wifi hotspot.

So at home, or a trusted family members house, behind a Firewall is okay?

As long as any wireless network is encrypted, yes. And any wired network should be safe.