Just out of curiosity, if you haven't logged on your BTC client 3 months in and received a payment a week after you stopped using the client.
Wouldn't it be possible that if we didnt download the full blockchain the client's wallet wouldnt be updated properly?
Sure. The wallet will be updated when it reaches that part of the blockchain. However that doesn't change that your balance is there, even if your wallet doesn't reflect it. You just need to give it time. You would still be able to check your balance with blockexplorer.com , for instance.
I think I mistook your post. I thought you meant that if the client were to download the latest N blocks, and lets say where N == 200. That the wallet.dat would update even if the transaction was 1000 blocks back. I know that the wallet balance should be a certain amount -- if you look at block explorer -- but if the client were never to download that part of the blockchain how would they be able to spend it?
Such a client would validate transactions but wouldn't know your balance from your blockchain alone, it would need to check elsewhere like Electrum does. You can use blockexplorer.com to check your balance. Or blockchain.info .
To know your balance for sure, the client would need at least the tail of the blockchain since your address first received a transaction. Then, if you only care about a number of addresses, there are tricks to compress the blockchain further but it gets complicated and you cannot manage more addresses afterwards. But normally, a new user starting just now and only managing new addresses won't need all the blockchain for genesis for any normal operation including checking balance (we are assuming his addresses are new and have no balance when he first installs the client). Maybe such cases should be covered as an option during install.
