This can already happen, say, if a person has a copy of their wallet on their Linux partition and another on their Windows partition. If you spend the money on the Linux partition, and then a few days later boot to Windows, it will appear you are richer than you are until the blocks all download.
... and that's exactly why I discourage people from doing things like that. It is too easy for two "copies" of a wallet to get out-of-sync.
You have to be a geek and muck around with copying the wallet.dat file from one place to another to get into trouble, and that is by design. I have no problem at all with geeky tools that let you do dangerous things (like PyWallet).
The JSON-RPC interface is trickier, because adding dangerous functionality there might encourage web services to do not-so-smart things like sending private keys over unencrypted/unprotected channels ("Email the private key as a Christmas gift" works great for a while, and then the bad guys start looking for privkeys in traffic they're sniffing and spend them before your Dad can....)