Do you really want me to hack your sim? :-)
Apparently it is possible to hack a sim but only the older type data encryption standard. Cards running the new triple DES are not affected.
The hack can be carried out in about two minutes it seems, by cloaking an SMS so that it appears to come from a carrier. In about a quarter of the cases the hacker will receive an error message with the necessary data to work out the SIM's digital key.
When they have that information, another text can be sent which opens it up so that the hacker can listen in on calls, send messages, make mobile purchases and steal all manner of data.
If you are worried about your sim being hacked it would be a good idea to ask your network provider for a new sim, you will be able to keep the same number but it should be safer.
I hope that helps.