As a follow-up, someone posted
this screencap of an e-mail from Gabe Newell who stated that all card information is encrypted using
AES256; while AES256 does have some
known attacks, the cipher is secure, and assuming the e-mail is indeed valid even a best case attack would still require 8.5x10^37 operations to complete. That's about one magnitude fewer than the number of available (albeit not used) IPs in IPv6. If you converted each operation to a mile, you could travel back and forth between here and the Andromeda galaxy about 348,000,000,000,000,000,000 times.
Also, it should be noted that the attackers will likely never see the credit card information in their lifetimes (or their children's lifetimes or their children's children or...) barring 1) any private keys used to decrypt the information are still secure and 2) there's not some unexpected advancement in quantum computing that greatly reduces the efficacy of the AES cipher.
TL;DR Steam hasn't yet pulled a Sony.