The device= overrides do not seem to affect the panel rules.
When I use device=mobile, I get redirected to my mobile domain, the right theme gets selected, but I see the panel for the desktop device.
As far as I can tell, this is related to http://drupal.org/node/893378 but for the 2.x tree and with a different solution.
I believe mobile_tools_is_mobile_ctools_access_check() in mobile_tools/plugins/access/is_mobile needs to use mobile_tools_get_device() instead of mobile_tools_is_mobile_device() on line 62.
Making this change appears to work for me, but I am still testing it. I will try to generate a real patch, but this isn't totally straightforward for me on this particular site install.