I was messing around with Final Cut Pro 5.0 today after installing FC Studio. I did a get info on the app and noticed a Tab saying 'Plug-ins' between 'Languages' and 'Ownership'.
I had some free AE plug-ins which I wanted to check with FCP 5.0 anyway, so I downloaded them to my desktop and then clicked add from the Plugins tab in the Get Info window for FCP.
This allows you to choose the plugins and then they are added to the FCP bundle. Sure enough they are loaded with FCP. Also you can then go to this Info menu and Disable of remove plugins one by one very easily which is very useful if some plugs are causing issues!
This appears to work with FCP 4.5 too, I think its just a Tiger only feature.
Mark
By the way the free AE plugs are worth checking out (not sure how well they work with FCP 5.0 yet):
Dragonfly