This not necessarily caused by the plugin itself, but it should happen in other parts of your application too if it's not caused by the plugin. I have used this approach many other times and it worked. I assume Categories is extending Zend_Db_Table_Abstract.
A short checklist:
If plugins work normal except when using the DB component , it's not a plugin problem.
Do you have any adapter registered via Zend_Db_Table_Abstract::setDefaultAdapter somewhere in your index file?
Do you have the correct includes? Meaning that your model dir is in the include path?

Debug tips:
Try running the framework with E_ALL | E_Strict to spot any easy to find problems
Try running provided unit tests that come with the framework to spot any problems that you may have in your environment
Try using something like APD to debug your app to see where it crashes ( apd is really easy to use)
If your model works in other part of your app , register it in Zend_Registry and fetch if from there (this is dumb , it should not work if it doesn't work in your plugin either)
Regards