Sirius has a unit which has an optional add on to make it portable.  The
Xact XTR-1 is a plug and play Sirius satellite radio receiver and also has a
boom box option.  This unit is cheaper than the MyFi's $350 price tag and is
available now.  Here's a picture of the Xact with its battery pack for
portability attached: http://www.xactcommunication.com/itempics/18_large.jpg

Check out www.xactcommunication.com for more information about the XTR-1 and
related accessories.

I'm quite speculative about the MyFi unit and it's integrated antenna.  I
have a feeling that if you're not in a metropolitan area which would have a
terrestrial repeater nearby, you might have a hard time keeping a signal,
especially if it's attached to your hip like a walkman.  They have a feature
in the MyFi that records 5 hours of radio internally so that if you're
somewhere where you cannot get a signal, you can listen to the recorded
stream.  However, you cannot record while it's in the car kit from what I
hear, not that you'd want to anyways since you'd just listen to the same
thing you heard on the drive there.  I can't get a signal inside my
workplace.  I'm at work 9 hours a day so this wouldn't be enough recording
for me.  I listen to Sirius' online streaming, which XM has, too, but they
charge extra for it.

XM is ahead of Sirius in chipset generations.  XM is on Gen 3 chipsets where
Sirius is on 2.5.  Sirius doesn't expect to release 3rd gen equipment until
around this time next year.