I foresee that most people will not have bitcoin installed. Instead, they will have an account in a bitcoin-bank. Thus, it is very easy to have a debit card. It would works the same way it is working today: tell your bank that you spend some money in favor of vendor X. Vendor X receives OK and trust the bank to have sent the money.
For the most advanced scenario, I've written this (in french) in the following little story: http://ploum.net/post/futur-economie
The way it works is very simple:
- The vendor gives you a QR code which embed a bitcoin address.
- You scan the QR code with your phone and have to enter a pin code and confirm the amount
- The bitcoin address is sent to your bitcoind, which could be either your bank account, your remote computer or a local wallet on your phone, it doesn't matter.
- The vendor see a green "confirmed" as soon as the transaction have been confirmed enough (which depends on the price)