One of the changes in Tiger is that if you have set the Finder to show all file extensions (via the Advanced panel in the Preferences window) then you see the .app extension for applications.
This used to be a special case in earlier versions of the OS: you wouldn’t see the .app extension.
I understand the change, because I understand the desire not to have special cases. (That’s not to say I agree with it. Maybe, maybe not. I haven’t formed an opinion yet.)
It did trigger a bug in NetNewsWire in Tiger: if you click Post-to-Weblog, and you have the Finder set to show all file extensions, then it wouldn’t work. Your weblog editor would launch but it wouldn’t open a new document window.
The next build of NetNewsWire will fix the bug, of course. In the meantime, if this is a deal-stopper for you, you can temporarily work around the bug by telling the Finder not to show all file extensions.