I made a file for Atrix's CM9 based on this thread.
I didn't post a screenshot cause you can see it at link.
Every UI will change to that of tablet, and every app will act as tablet.
So here's the steps you have to do.
1. Download services.jar and overwrite it to /system/framework.
(Of course, need 644 permission.
If you are not sure, see other file's permission in folder.)
(This file is based on alpha 7)
2. Open /system/build.prop
3. Find the line 'ro.build.characteristics=default'
and change the 'default' to 'tablet'
4. Set the screen density to 120 by changing
'ro.sf.lcd_density=240' to 'ro.sf.lcd_density=120'
5. Save and reboot.
6. You will get tablet UI
So, If you want to get back,
just change your density to 240 and it will revert to phone UI.
We need reboot between UI change now,
but maybe devs can do some mod after CM9 have all feature