One guy called Jonathan Fisher did come up with a solution, but his page disappeared off the web a while ago. But I managed to find it using the Wayback machine to get the archived webpage.
Basically you need to first install the Blackberry JDE (which is Eclipse based), then in Netbeans Mobility settings go to Manage Emulators and do Add Plaform in the platforms manager to add a custom platform that points to your Blackberry JDE directory, and remove all the automatically added jars except for net_rim_api.jar
Next, when you start a MIDP project, select your new custom Blackberry platform as your emulator in the project settings. You will also need to add some custom build settings to build.xml, plus a Blackberry .alx application loader file to the project. It's a bit long-winded, so best to follow the instructions as per the aforementioned link.