At least one version of this scheme will open up with the line, "Hello, thank you for shopping at the Blizzard Store!" according to a post on Siliconera. The first prescribed step in the email asks the user to create a Battle.net account at a URL that doesn't belong to Blizzard.
Senders' email addresses can be faked, so just because your message claims to come from Blizzard doesn't always mean that it's the real deal. Free keys or keys that come bearing receipts for purchases you didn't make are also good indicators that your email is probably a phony.
Ultimately, it's never a bad idea to check out Blizzard's Battle.net security page to make sure you're taking the proper precautions, and if you think you may have already made a terrible mistake and left yourself vulnerable to hackers, Battle.net has a response team set up to aid you in dealing with that as well. Malware in "StarCraft 2" may only be a problem with illegally pirated copies, but emails phishing for passwords have been circulating, and Battle.net users should be on the lookout for fraudulent messages that come bearing fake product keys. The scam will insert a malicious URL in place of legit Battle.net login and proceed to request personal information, so be sure to vet any unsolicited email claiming to be from Blizzard before clicking through to any unsavory links.