Mike Calligro from Microsoft's Windows Mobile product group expounds on why the "X" in the PPC OS does not actually close a PPC application. If you'd like to weigh in with your views you can read the article here:
http://blogs.msdn.com/windowsmobile/archive/2006/10/05/The-Emperor-Has-No-Close.aspx
Please do, the Windows Mobile team would like to hear from you.