I understand the paranoia with a new (and clearly not even vaguely understood by most of the forum users here) technology... so please don't take this snarkily. But, I'm imagining the steps necessary for somebody to steal fingerprint data from your device, when that data is ONLY housed locally AND the security of which has been independently verified as meeting or exceeding security requirements of the major world banks. This isn't like trying to figure out a jailbreak, people...! Imagine if there was only a single point of entry to try to jailbreak & Apple didn't manage that, a 3rd party company of security professionals did. Umm, jailbreaking would become a thing of the past.
Do I believe that there is SOME incredible genius hacker out there that, given enough time and resources could crack even the super, super high-end banking security level encryption on a forthcoming iDevice and possibly (and ONLY if they physically had your device in their possession) then figure a way to extract your fingerprint data to an external machine.. and then they'd have the job of trying to replicate that data, somehow grafting it onto a living finger to trick said device into thinking it is you? Sure. There may even be a small handful of hackers in the world of this caliber. To think that they are going to go through that Herculean level of effort to have the brief ability to check your Facebook posts & possibly order up a few iOS games linked only to your account is bizarre and laughable beyond the "tin foil hat" conspiracy theorist level, and more like the "I belong to the Church of the Flying Spaghetti Monster" kind of insane.
Remember that, even in the extreme "a super-hacker followed me & boosted my phone!" scenario I just outlined, they would easily be foiled no matter what if you noticed your phone was gone and either performed a remote wipe.. or simply changed your password prior to them getting it back to their secret lair to uncover your deep, dark secrets that you clearly house on your iPhone...