Based on the open-source BTstack project, it's been tested on:
But should work on any similarly functioning device, and the status for iPhone 2G appears to still be up in the air.
Until Apple sees fit to add their own BT driver (and the HID, Human Interface Device profile) for their own BT keyboard, this is another area only Jailbreak can fill. So, as always, if you try it out, let us know how it works for you. Preferably, let us know while typing on your iPhone via a BT keyboard!