Even without a SIM card, you can use an activated iPhone 4.1 as an iPod Touch music device. Although you can't make phone calls, send text messages or browse the Internet over a 3G network without a SIM card, you can still take advantage of much of the phone's functionality. This is particularly true when you connect the device to a Wi-Fi network, which gives you the ability to browse the Internet and download purchases from the iTunes and App Stores.
Activate the iPhone 4.1 with a SIM card from a supported cellular carrier. If the device was previously registered with a mobile carrier, you don't need to repeat this process unless you suddenly see the "Connect to iTunes" message on the iPhone's screen.
Connect the iPhone 4.1 to iTunes with a USB cable to complete the activation process.
Remove the SIM card and begin using the iPhone 4.1 as an iPod Touch.
Upload music, videos and applications to your iPhone by syncing the phone with iTunes. If you have access to a wireless Internet signal, you can also purchase and download data directly on your phone.
Browse the Internet with the Safari Web browser when connected to a wireless Internet signal. In Safari, you can search the Internet and view most Web pages as you would on a computer. As long as you're connected to a Wi-Fi network, you'll also be able to send and receive e-mail messages on the iPhone 4.1.
Take photos and videos with your iPhone 4.1. To get them off the device, either e-mail the photos over a Wi-Fi network, or connect your iPhone to iTunes on your computer.