That email wasn't a phishing attempt or some attempt to trick you; Adobe revealed late last week that they had suffered a security breach which affected up to 2.9 million customer accounts:
Cyber attacks are one of the unfortunate realities of doing business today. Given the profile and widespread use of many of our products, Adobe has attracted increasing attention from cyber attackers. Very recently, Adobe’s security team discovered sophisticated attacks on our network, involving the illegal access of customer information as well as source code for numerous Adobe products. We believe these attacks may be related.
I haven't bought anything from Adobe so I haven't given Adobe any credit card info, but I still got an email on Saturday. I am taking it seriously, and so should you.
If you have ever downloaded a DRMed Epub ebook and then transferred it from your computer to another reading app or ereader then you have probably used your Adobe ID somewhere in the process. This ID consists of an email and password, and it is this info that may have been compromised when Adobe's server's were hacked.
And now that password may or may not be in the hands of a hacker. This is no reason to panic, but a prompt response is warranted. I suggest that you read the email from Adobe and follow the instructions. I don't think it's very likely that your financial info will be compromised but I also believe that it is better to be safe than sorry.
Update: And while you're at it you might want to also consider whether you also used the same password/email combination on other sites. You should change those as well, especially if they have credit card or other payment info attached. Thanks, Alexander, for the suggestion!
P.S. I know that many in the anti-Amazon crowd like to talk about how the Epub ebook market is not locked down to a particular ebookstore, but the reality is that all the ebookstores, apps, and ereaders that support Adobe DE DRM all have the same weakpoint: Adobe. Tell me, how exactly is this better than being locked to a single ebookstore?