But I use my iPhone 4 as a WiFi hotspot primarily for my iPad. I use MyWi, which is a Cydia (jailbreak) app and it was $20. But it works great.
It can not only turn the iPhone into a WiFi hotspot, it can also tether to a laptop or other device via Bluetooth.
Can tether a pc/mac via USB too which is less draining on the batteries in both. Or it can share its WiFi Internet connection to a pc/mac via USB. Might seem odd to do this but there are places where my iPhone gets free wifi whereas my laptop would not (e.g., at&t hotspots). So I can get the WiFi on iPhone and then share it with laptop via USB. I have even done that and then shared the USB internet connection on my laptop with its wifi connect to feed my wifi only iPAd. Yes that sounds convoluted but I did it and it worked.