Tom,
This is likely a problem with OSGi/eclipse not yet recognizing Mountain Lion. Once eclipse supports it we will be able to. However, note that the REST explorer is not currently available on Cocoa installs; have you installed the Cocoa or the Carbon version?
When you say that the Rest Explorer doesn’t think it’s a Mac OS-X, where exactly are you seeing that message and what it the exact message?
As a final note, I guess it comes as no surprise that we can’t offer full support for unreleased operating systems, though we’ll do our best.