Is there a standard wallet format that can be read with different bitcoin clients, and that is likely to be compatible to future bitcoin clients as well and thus is usable for long term storage of bitcoins? Or how am I supposed to transfer bitcoins between different bitcoin clients/computers/client versions?
There is the Wallet Import Format, which should be a standard for Bitcoin wallets. As to how popular it is, I don't have any data on it. Alternatively, storing the raw private key should suffice, as it can be converted to any other fancy format whenever needed.