OK, I think I got that too. So scanning the entire block chain is only necessary if, say, you want to know the balance for an address that is not your own? Even then, you probably can scan backwards from the current block and stop when you've found all generation transactions.
Florian
or if you think the client missed a transaction announcement (e.g. internet outage, power outage, or something else that removes you from the network for a considerable length of time) and somehow missed it when you came back up and pulled the intervening blocks. that's what the -rescan flag is for. it forces the client to scan through the whole blockchain looking for transactions involving an address/keypair that you have in your wallet file.