Using my current 3G Sim with the iPad
The iPad with 3G is sold unlocked. In the UK, the iPad with 3G will work with Orange, Three, O2, and Vodafone.

To be pedantic, the iPad's 3G connection will not work with carriers such as T-Mobile USA, which in part uses the 1700 MHz frequency for 3G. The iPad with 3G supports the necessary frequencies to fully work in the UK with Orange, Three, O2, and Vodafone, and SIMs from these carriers can be swapped seamlessly.