The FroYo/GB OS can move applications to the area of the phone named "SDcard." On some phones, this is the card you put in.
However, as I've tried to explain. The Atrix considers the INTERNAL storage "SDcard" and the removable SD card you put in "SDcard-Ext."
See:
When you Move an app to the SD card on FroYo/GB, the operating system looks for it in "SDcard" - and on the Atrix, that's the internal storage. The FroYo/GB os builds do not know how to handle apps stored in "SDcard-ext" named storage.
You would have to re-write the firmware of the Atrix OS builds to have it change how it names the storage area to do what you want.
And, it's the same way on my other Android devices that have an internal storage area. Take my ViewSonic Tablet. It also has 16GB internal storage that it see's as "SDcard" and that's where it moves apps to. It calls the external SD card in the slot "SDcard-ext." And, it can't move apps to "SDcard-ext" either. - So, this is not an Atrix issue. It's how the Android OS builds see the storage areas of the devices.