Turning my old iPhone 3GS into a regular iPod Touch

I have an older iPhone 3GS that I want to use as an iPod Touch. It no longer has a sim card in it and I had to restore it to the factory settings. Now when I plug it in to start it up, the phone says it is locked b/c it can't be registered without a sim card. Any ideas as to how to get past this so that I can use this as an iPod Touch?