NoMad,

  I had the same question about my Sanyo 6600 and spoke with Sprint
about it today.  Apparently the $15/mo Vision service does not include
the phone as a modem option.  That is only available on PCS Power Vision
plans.  That being said, it also depends on your phone.  Until I got the
 6600 last week I had been using a PM-8200 and using the phone as a
modem  with no problems on a $10/mo Vision plan.  I also asked Sprint
about this and they said that the older phones did not distinguish
between surfing the web via the phone or from a PC via a cable, so the
PC could surf fine.  The newer phones DO know the difference and will
block the ability if you don't have a Power Vision plan or the $39/mo
data plan.

I would think the 8400 is old enough that it won't know if you are
surfing the web via the phone or a connected PC.  As to the USB drivers,
yes the standard Sanyo USB drivers should work fine.  As long as your
phone shows up as a USB modem.