Let's that we have roughly the same client, with a little twitch. There's no private key in wallet.dat. The client can still accept payments and display the balance
Now, when the user wants to make a payment, the transaction is prepared and sent to the phone (say, via bluetooth) to be signed. A dialog is displayed to the user on the phone, with the destination address and the sum to be transferred, and he is asked to confirm/reject the transaction.
Benefits: No need to worry about wallet.dat. Also, maybe a way to make purchases in brick and mortar stores.
Doubts: Are files on phone safe(r)? I'm worried about trojans, mostly
Malefits: (if that is a word) Obviously more complex. Also, just an idea.
. But the way it may be a good idea to have such a restricted client in POS