Orange has announced via Twitter that it will be giving microSIM adaptors for its iPhone 4 customers for free, one which allows the diminutive card to be converted to a regular sized one.
Customers will therefore be allowed to take out their SIM cards and use them in other smartphones if they wanted to. You could, in theory, take out the microSIM out of your iPad and put it in a phone (or a hybrid device like the Dell Streak), although this may violate your terms and services.
O2 has also offered SIM adaptors to its end users although they have to collect it instore. That said, we're still not sure why you'd like to use a microSIM card into a normal phone except if the operators want/expect you to experience with the iPhone (or any device that uses the microSIM).
The iPhone 4 will be released on the 24th of June and the three T's (Tesco, T-Mobile and 3 UK) have yet to publish their tariffs for the device although that shouldn't be long now.