I just created an XM account and selected a plan, then cancelled out before entering credit card information...mainly because I was irritated to see the late disclosure of an activation fee, and also from curiosity to see if they will offer a discounted deal.
For reference, the activation fee is $15 and the Premier plan is about $17 per month plus <$3 tax/fees. It appears to be possible to subscribe a month at a time, although they do offer discounts on quarterly and annual plans.
A forum devoted to XM is at
http://xmfan.net/viewforum.php?f=2