I may be asking a question with an obvious answer, but could a US citizen immigrate to Panama (or another CG-free country), then cash in his bitcoins tax free, and be off the hook for taxes in the US?
You have to irrevocably renounce your citizenship. With the caveat:
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Persons who wish to renounce U.S. citizenship should be aware of the fact that renunciation of U.S. citizenship may have no effect on their U.S. tax or military service obligations.
https://travel.state.gov/content/travel/en/legal-considerations/us-citizenship-laws-policies/renunciation-of-citizenship.html
So I'm guessing if you haven't realized any gains until after renouncing, you're safe?
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(contact the Internal Revenue Service or U.S. Selective Service for more information)
Ha!