In the inteview packet, it says:
Please note that the Stanford environment is a casual one. You do not need formal clothes for interviews or for the dinners (which are sometimes at local restaurants or at the homes of program faculty).
I don't know how to interpret this. I can't decide if it means a suit isn't necessary, but you should still wear the standard business casual, or whether it means that jeans are okay. I don't mind wearing business casual, but I don't want to be the only one!