Mark is exactly right: Gatekeeper is a new Apple feature designed to increase security on Mac OSX. It does not limit you from running applications from outside the Mac App Store if you wish.
credneb wrote:Nisus Writer Pro is now signed using an Apple Developer ID, so it runs under Gatekeeper (OSX 10.8)
Could you please explain what this means?
The link dshan gives explains this in more detail, but briefly it means that you will be able to run NWP under Gatekeeper, whose default settings only allow running apps downloaded from the App Store or apps signed using a "Developer ID". Now that NWP is signed using a Developer ID, a user can download it from our website directly and still have it run on 10.8 / Gatekeeper.
Will future updates and upgrades have to be acquired through Apple?
No longer able to simply give our money to Nisus directly when needed?
You will definitely be able to continue purchasing NWP directly from Nisus Software directly, without using the Mac App Store.
One thing that does need to be mentioned is iCloud support: Apple only lets apps distributed via the Mac App Store integrate iCloud. So any iCloud features we may add in the future would only be available in the Mac App Store version. That's not our decision; it's something enforced by Apple. Luckily this appears to be the only restriction we see forthcoming at this point.