Where are bitcoins stored in the first place? Are they spread out in the mist of blockchain ledgers across the globe, or are they localized to the positions where the private keys reside? If they are stored in the global blockchain, then are you legally required to disclose these "foreign account balances"? If they are stored in the local private key, then are you required to disclose your brain wallet when you cross an international border? Maybe it's a bit of both.
Bitcoin motivates one to rethink what money actually is. Ensuring 100% compliance with legacy financial and tax rules would require disambiguation of what a bitcoin is to the point of logical contradiction, I believe. I expect if bitcoin adoption continues to grow, laws and regulations will bend to bitcoin rather than vice versa.