Tesco have recently released an iPhone app for Clubcard. Simply download the app from the itunes store and enter your clubcard number, when at the Tesco checkout in store just click to open the app and hand it to the cashier for them to scan.
It takes a little getting used to, having been used to the plastic clubcard card or the keyfobs. It certainly confused a few of the checkout staff who are not used to the app yet! I've handed over the phone to a look of bewilderment on a few occasions!
Here is how it looks on the iPhone