I'm not sure what you mean about a medical Dictionary plug-in. Are you referring to NaturallySpeaking? If so I've read a bit about this at [Edit: Link Removed] but again I'm not sure what you're referring to.
In Mac OS X Leopard, there is an application that comes with the OS called Dictionary.app that is actually quite useful and pretty easy to use.
As is, this dictionary application contains The New Oxford American Dictionary as a "plug-in" file (or reference library), and the application is capable of having additional dictionaries (or references) "plugged-in". For instance, there is a Japanese-English Dictionary and a group has recently put together a Chinese-English Dictionary. There is also a thesaurus available. My original question was whether or not anyone has thought about putting together a medical terminology dictionary (because the New Oxford simply doesn't cover a significant amount of medical terminology) or knew of one that was already available. In Leopard, with developer tools installed, you have the capability of putting together your own reference library. I figured with enough interested people, we should be able to cobble together something useful.