When I first got my iPhone 4, I tried tethering via BlueTooth, but was unable to get the BT pairing to go through, so I forgot all about it. My 3G modem gives me all the coverage I want, and with my MacBook Air I can share the connection with my (WiFi-only) iPad. Today, however, I was nudged to try the iPhone 4 tethering, and this time everything worked flawlessly. Right now I’m composing this on my tethered MacBook Air.
Of course, all of this “tethering” stuff feels antiquated compared with the mobile hotspot features on the latest Android phones, but it’s good enough for now.