This may be a daft question.
I have just upgraded to a Blackberry Curve, from a Blackberry Pearl. (On Orange)
I havent put in the new sim as yet, as I am expecting a call, and it will take an hour or so to be activated. (from previous experience)
Do I actually need to put the new SIM in? Will it make a difference to the funtioning of the Curve?

no, but if you dont put it in and activate it, from experience, orange will bug the crap out of you until you do