It's not that it can't pair really, an iPhone can pair to a computer in tethering mode as others have mentioned... it can even pair with other iPhones, there are Bluetooth "Walkie Talkie" apps in the App Store.
Apple has chosen not to implement some of the Bluetooth profiles, such as file transfer profile. Currently they support only HSP/HFP (Headset Profile / Hands Free Profile, for phone calls) - A2DP/AVRCP (Advanced Audio Distribution Profile / Audio Video Remote Control Profile) for stereo music, HID (Human Interface Device) for keyboards, and perhaps a couple of the lower-level messaging profiles. Internet sharing most likely won't be added as a generic Bluetooth feature since that is already addressed with the hotspot capability.
There are
a TON of available profiles, most of which Apple doesn't support.
They do add profiles from time to time (the hardware is capable of most, if not all) - for example when I got my iPhone 3G, it did HSP/HFP - then (iOS3) they added A2DP - then at some point (iOS4 maybe) they added AVRCP. If there is a particular Bluetooth capability you want to see added,
tell them about it.