iPhone 3G/3GS comes with built-in Bluetooth radio, that's version 2.0+EDR in iPhone 3G and version 2.1+EDR in iPhone 3GS. Although the radio specification is slightly different, Bluetooth in iPhone 3G and iPhone 3GS work in the same way since both use the same iPhone OS.
Bluetooth in iPhone works somehow in a "protected" mode. One can not freely exchange image/photo files or other objects with other Bluetooth phones or Bluetooth-enabled computers, commonly a fun thing to do with other phones because this iPhone (Apple) implementation of Bluetooth does not support OBEX (object exchange) or OPP (object push profile) and FTP (file transfer profile). Even for sending/receiving photos to/from other iPhone, one needs the help of an external app, such as Bluetooth Photo Share. (Personally, I used this app for transferring some images from an iPhone 3G to an iPhone 3GS and it's quite helpful for bypassing iTunes procedure which would take longer time.) Many consider this as major drawbacks compared with other average Bluetooth cell phones, but I think it was designed to be just like that.
However, it works very well with a third party (mono/stereo) Bluetooth headset even though -in my experience- iPhone needs longer time to discover a Bluetooth headset in its vicinity compared with other Bluetooth smartphones.
Bluetooth headset can be used while placing/receiving phone calls, playing audio/video files (mp3, flv, and other supported formats), recording/playing voice memo, and working with third party audio apps.
It has been said everywhere that iPhone 3G/3GS running iPhone OS 3.0 or later can be used as a Bluetooth modem. Well, actually iPhone works as a PAN NAP (Personal Area Network Network Access Point), therefore it is capable of connecting more than one Bluetooth-enabled computers to Internet simultaneously and enabling file/printer sharing between them. This is an advantage over other smartphones if users know how to use it. Have you ever heard or used this wireless networking access point/router application of iPhone 3G/3GS before?