How do I change the default SMS application on my HTC Hero?

I recently got completely fed up with the built in Messages (SMS) application and switched it out for Chomp (also looking at Handcent, haven't decided on one yet). I want to use the "Messages" icon though, as well as treat the new app as the default SMS program. I know about "Manage Applications" and clicking the "Clear Defaults" button for the app. But the "Clear Defaults" is disabled for the Messages app and it says that no defaults are set. I read on [someone else's post](https://android.stackexchange.com/questions/2394/can-i-make-ebuddy-be-the-default-instant-messanger) that their AT&T provider had somehow hardcoded this as the default app and they couldn't change it. So... Can they really do that? Is that why my HTC Hero doesn't let me clear the default? Is there a way around that?