According to MacRumors (source), the new Gatekeeper feature of the upcoming Apple OSX release (v10.8, AKA Mountain Lion) will require applications not distributed via the Apple-curated App Store to have a "personalized certificate" granted by Apple before they can be run under the new OS if the Gatekeeper security features are active. However,
For users on the default setting, they can bypass the initial Gatekeeper check the first time they launch an unsigned third-party app by right clicking on the app itself and choosing the "Open" command. Once the application has been opened one time, Gatekeeper no longer has any control over it.though this clearly circumvents the benefit of Gatekeeper.
Since the Second Life viewer and the related third-party viewers (TPVs) are not App Store items, Linden Lab and TPV authors may have to address this issue during the download phase, apply for certificates, or submit their wares to the App Store. Timeline? Soon, I would say, as the release of the new OS is in public Beta and Apple seems to be preparing for press events next month.