Well, ... all the blocks solved by your island would be orphaned once exposed to the main net because the current capacity of 20 Thashs/s has way more blocks solved than the island fork. But the client wouldn't care, it would see the reorg and notice the transactions were back to being unconfirmed and would broadcast them again. Presuming no double spending occurred then the transactions would be relayed and then included in blocks. So rejoining is not a problem, as long as there were no dishonest who spent on the island and also spent those same coins on the main net as well.
The graceful handling for reorg is a good point I hadn't thought of, but isn't there still a problem with newly-generated coins contaminating many of the transactions? If some of the coins in a transaction don't "exist" on the mainline chain (or at least not in the same coinbase from the island), they can't be spent on it. (assuming the island can't make a longer chain than the Internet's chain.)
Then the desert islanders simply don't create coins - i.e., they don't mine. If they did mine and generated coins, they'd also have the problem of any transactions in the blocks they mined being reversed once the main blockchain overwrote it. So, mining would be pointless and only add confusion in such a situation.