Better bitcoin wallets

Simon Barber published a paper on improving bitcoins as a currency.  This section, taken from their paper, mentions improving usability through user wallets:

Super-wallets. To address the usability concern, we propose the simple idea of superwallet, i.e., a user’s “personal bank” where most of her coins are stored. The superwallet is split across multiple computing devices, using threshold techniques as above.  In addition, the user carries a smallsub-wallet with her on her smartphone.

Pre-approved transactions are setup so that the user can withdraw money from her super-wallet onto her sub-wallet, periodically in small amounts (similar to how real banks let people withdraw cash from ATMs today). The user now only needs her smartphone to spend money in her wallet, and in case her smartphone is captured by an adversary, the user only loses the small amount of money that she has in her wallet, but not that in her personal bank.  

Large amounts can always be spent from the super-wallet using a threshold of devices.  Both approaches can be implemented as backward-compatible and incrementally deployable wrappers, requiring changes in the signature generation but not verification.