Since both the iPad and the soon-to-be-launched iPhone 4 use the new MicroSIM standard, you might be interested to know why Apple has switched to this new format.

The iPhone User Guide has a good summary and it basically boils down to three main new features:

1. MicroSIMs allow for multiple simultaneous applications accessing the card through logical channels;
2. MicroSIMs support mutual authentication as a way to eliminate carrier spoofing by allowing the SIM card to authenticate the cell tower to which it is connecting;
3. MicroSIMs add a new PIN protection with hierarchical PIN management with a universal PIN, an application PIN and a local PIN.

So while the smaller chip looks similar to the old one, the difference is more than just less amount of plastic surrounding it. Don’t try to cut the plastic around your old SIM into a MicroSIM format because it just won’t work.