Windows Mobile for Smartphone is a separate operating system from Windows Mobile. Both are derived from Windows CE, so they're closely related. However, the user interface is different. For example, the Smartphone version is written for devices without a touch screen, so its programs have to be navigable using only keys (as opposed to using a stylus).
Right now, Stackz is only available for Windows Mobile, not Windows Mobile for Smartphones. However, given that the two operating systems are essentially the same under the hood, I'm hoping that a port might not be far away. Maybe?