That is a "per phone" option.  I have the unlimited on my Treo but the
other four phones on the plan have no media access other than "per use."
This makes sense since if you had the unlimited you could, conceiveably,
be going nuts with surfing on five different phones.

What kinda surprised me is the medianet package is strictly a monthly
thing.  You can turn it on and off and adjust it at will (though I
suspect they will clout you with some service charge for the changes to
the account), i.e. you can have unlimited in May, and then cut it down
or off altogether for June and then come July have the unlimited again
if your needs change.