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I am a practicing attorney at a law firm, and I am contemplating applying for a federal clerkship. For obvious reasons, I would not want to disclose my future employment plans with my current firm. In the event I am offered such a position, are there any ethical prohibitions to me appearing before that judge prior to the start of the clerkship? Likewise, would the judge need to disclose my future employment with him to the opposing attorney?

Reference to a federal ethical opinion/canon/etc. would also be helpful.

Nothing on point to your situation in the Rules of Ethics, but Canons 4C(4) and 3F(1) are a good starting points. But if you are extended an offer, it will be up to the judge on how he or she would want to handle your situation.