For developers with Real Studio, well, I can recommend you read about how to code sign your applications. With the $99 Mac developer program you get already today the certificate so you can sign your applications like you do for the App Store.
Because with 10.8 will have default setting to allow only new applications from Mac App Store and identified developers. So if user downloads your unsigned app from your website, they get a warning or the OS will not open app. Yes, users can change the policy or allow your app, but default is to show a warning. You certainly want to avoid that and sign your app.
Read more on Macworld.com.
Also Xcode has a view changes. I have to check them later, but it's possible that on our plugins we may loose 10.4 PPC support, 10.4 support or 10.5 support. That depends on whether older PPC compilers still work on 10.8 and whether Apple decides to drop older deployment targets.
When release date is set and we have final SDK, we will start doing those plugins and have them ready with 10.8 shipping. Due to NDA with Apple we are not allowed to give away plugins with new features before that.
For Filemaker developers it may also be required to sign Runtimes. But we have to wait until we can verify that. And maybe Filemaker 12 has features for that to sign automatically.