apple
iPhone app lets you hail a cab from home
Basically all the user would have to do is register an account and link their credit card to that account. After selecting their destination, a list is supposed to be generated where a a variety of cab companies can be chosen, all with apparently different pricing and ratings, so as you can see in the picture to the right, the cheaper cab company has been given 2 stars, while their more expensive competitors are given higher ratings (you get what you pay for).
The GPS built into your smartphone apparently tells the cab company where exactly you are and where to send the taxi to. Users apparently get the choice of paying by cash or using their iPhone, which is linked to their bank account, just in case they find themselves short on change. Not to worry Android and Blackberry users as the company apparently has plans to develop the app for your platforms as well.