The new Mac OS X Mountain Lion has an added security feature, Gatekeeper, which protects users from malicious software. In order for Gatekeeper to work, users are by default allowed only to download software from the Mac App Store whose certificate has been digitally signed with a Developed ID provided by Apple.
According Adobe, it has "added the Gatekeeper signing requirements to our currently shipping applications. However, our legacy products created before Mountain Lion's Gatekeeper feature was available are not signed."
If you happen to have one of the unsigned applications, Gatekeeper's security dialog might pop up, warning you against downloading unsigned applications.