I have been cutting SIM cards for years but now with different thicknesses it can really do a number on your phone. After a while it will stop reading the sim (or any sim you put in) and say no SIM card. Warranty is definitely void if they find out you had a cut down one in the phone! For $10 get a new one!!
If you have cut your sim card down and have to send it in for warranty work make sure you take the sim out as the warranty is void if they see it. Just had a call about this and was told "No cutting of sim cards" which is something I've been doing forever!