If you have access to a server you can write some server-side scripts and use those to update a high score which the app can download. The level of security you need to wrap that in will depend on how popular your game is. The basic framework would be:
If you don't have access to a server, I don't know what to tell you. You can minimize the impact of a third-party high score application by having your app check to see if it's installed when you start. If it's not, ask the user if they want to install it and then send a market intent to take them directly to the app. If they don't want to install it, fire off an uninstallation intent for your own app, since it'll be useless to them.
