I don't think cash will diappear for some time yet, for a number of reasons:

a) It's not economical for shops to accept cards for small transactions, as they would barely cover the cost (something like 8p/transaction or 2.5%, it varies - which is why most shops won't accept credit cards for orders less than £5) of the charges levied by the card companies.

b) There's still a lot of cash (we're talking billions of pounds here) in circulation. It would take a long time to make a complete changeover - at the moment it takes months for the Bank of England to issue a new £5 note, never mind abolishing them altogether!

c) As some people have already pointed out, cash is anonymous (hence why it's often requested in ransoms - it would make it a lot easier for the police in such cases if all transactions were electronic) and people like this for various reasons, including paying builders and getting "a cash-in-hand discount" (i.e. the builders don't claim it as income). Okay, so that's not a legitimate example, but you get the idea.

d) Cash is a lot simpler to deal with than cards. Not only can anyone accept it (you don't need a special machine or anything like that) but it's a lot easier to spend as well (seeing as you don't have to mess around with signiatures). Also, could you imagine giving your card to someone selling items without a shop (e.g. the Big Issue)? Unless they had a wireless card machine (which could bring about all manner of new security issues), they'd have to take your word for it that the card was genuine and that your account was in credit.

e) Finally, cash helps you budget. If you have £100 in your wallet, you can't spend more than that, no matter how tempting it might be. With cards, you can easily build up a mountain of debt whilst remaining totally unaware of the situation you are getting yourself into.