We've spoken to Square-Enix regarding this matter, and they've laid the rumor to rest for us. The authentication tokens are not being discontinued (as they are pretty popular) and have now been re-stocked back into the store. We here at Massively highly recommend tossing the 10 bucks towards the token, if you can afford it, as it gives you access to the amazingly useful Mog Satchel -- a second inventory for your character. Better act fast, however, as these tokens fly out of the store very shortly after being restocked.
To purchase an authenticator, log into PlayOnline, go to the Final Fantasy XI top page, select optional services, and then click security token sales.