At the moment most sims are produced in a standard mini sim size.
This can be a major problem if you own an iPhone 4 or an iPad as they will not accept a mini sim.
Buying a micro sim often costs far more than a mini sim and choice is very limited, so it’s best to cut a sim down to size.
You can chance your luck carving it with a sharp knife & scissors or you can get a tool to cut it.
I have got myself a sim cutter from http://www.cutmysim.com/
It seems to do the trick and the sim now works in an iphone 4, and I hope to test it in an ipad.
The iphone4 comes with a handy micro-sim to mini-sim adapter which is shown holding the cut AAISP.NET sim.