It will not be cold storage, as in "private key is secure, by never being written to a computer system storage device and by being generated on a non-internet-accessible computer". Cold storage is a phrase used for the securest of offline Bitcoin addresses, stored as a private key or paper wallet, that are created in a way that the stored copy(s) of the address are the only ones that have ever been recorded.

A simple way to make your own is to boot off a non-persistent live CD with the ethernet cable unplugged, and run a copy of vanitygen to generate a simple phrase address (use a seed file with a couple minutes of keyboard pounding in it). Write down the resulting address/private key on paper, or dump 100 copies of it to your USB stick. Alternately, use a saved copy of the bitaddress web page to generate your address: https://github.com/pointbiz/bitaddress.org

What you are creating instead is a wallet backup. If someone steals or has already stolen your wallet.dat now or in the future, they would be able to spend coins received by the wallet in the past, present, or future, even money you send to "new" addresses. Address information could be recovered off a hard drive, even if the wallet file is deleted or the drive is repartitioned or reformatted insecurely.

If you plan on making a long-term backup, for the purpose of safeguard against data loss, after encrypting with a passphrase, you might consider starting Bitcoin once with a large keypool option such as bitcoin-qt -keypool=2000. This will fill the wallet with future keys that will keep your backup from becoming obsolete for a long time.