john smith wrote:

I would think, that as long as each unit had it's own XM subscription,
that wouldn't be a problem.

Each unit has to have it's own subscription to work in the first place,
just like a basic XM radio. Even so, on the radio side, XM forbids one
from using the radio they legally own, with the subscription assigned to
it in a commercial setting, such as background "Muzak" in a retail store
or restaurant.

This may be related to music licensing, ala ASCAP dues, etc... so it may
not be a factor for Weather data or onboard music in an aircraft.

Here's the music info from XM's FAQ:
http://www.xmradio.com/faq/faq_list_servlet.jsp?section_id=4&section_main=XM% 20Satellite%20Radio#154

Anyone with a 396 care to check the XM paperwork?