This is what is says on their forum;
TVCatchup is now compatible with the IPhone (or IPod Touch). To watch TVCatchup on your IPhone, simply go to:http://iphone.tvcatchup.com
There are currently over 50 channels available. The streams are optimsed to work on both 3G and WiFi, although it is suggested that you use WiFi.
Please remember an Uk Tv licence is needed for viweing however tvcatchup dont police this.
If anyone has any problems visit their forums, they will be happy to answer your questions they are such a nice bunch of people.
Also a word of advice with those using 3g make sure you dont go over your limits, even if you have unlimited! Best bet is to use wifi.