Oops.  I forgot they say that.  I think I've only use
a "real" T-Mo card once- a "free" $25 card came with a phone I
bought from T-Mo's website last Christmas.  While I wouldn't suggest
this is "proof" you can ignore the 90 day warning, I didn't redeem
that card until July- a good 6 or 7 months after I received it.

Honestly, T-Mo would have no idea how long those cards sit on store
shelves prior to sale, so I doubt they have any way to "enforce" that
threat.

I assume the 90-day requirement is a "CYA" thing to prevent someone
from whipping it out years later after T-Mo's plans have changed, or
they've been bought out by someone else, or who knows what else.

For example, I remember after I closed my Cingular dealership, I
found three old Southwestern Bell Mobile Systems Prepaid PIN
"activation cards" that had cost me $10 each.  I took them, and three
of my old TDMA phones to a Cingular corporate store to active them.
They told me those weren't good anymore, so I asked to trade them for
modern Cingular airtime cards since the old cards had no expiration
date on them.  After no one there could figure out how to accomplish
the exchange on their computer system, they agreed to just put a $30
"courtesy credit" on my postpaid Cingular account and throw away the
cards.  If SW Bell had had the foresight to print "card expires in 90
days" on those cards, Cingular could've stiffed me out of some money
(much like they often did when I was one of their dealers!) ;-)