Due to hit the Cydia store momentarily, Matthias Ringwald's BTstack Keyboard app allows users to type text into any iPhone application using an external Bluetooth keyboard. Built on the open source BTstack project, BTstack Keyboard runs a daemon in the background of any jailbroken iPhone 3G, iPhone 3GS or iPod touch with Bluetooth support. As you type text on the keyboard, the daemon generates synthetic keyboard tap events; the effect is the same as if you'd typed that text using the on-screen keyboard.
You will need to install BTstack and the BTstack Keyboard packages on a jailbroken 3G or later iPhone or 2nd generation or later iPod touch. The software has been tested with an Apple Bluetooth keyboard, a Think Outside Stowaway Universal keyboard, and a Palm Wireless keyboard. There's no reason to think it won't work with any standard Bluetooth keyboard, i.e. one that uses standard BT protocols.
With this small utility, users will be better able to take notes on the go using a standard keyboard in any text-based iPhone application. Yes, you'll have to haul around the physical keyboard, but the availability of folding on-the-go Bluetooth solutions makes this an exciting development for anyone who wants to expand their text entry possibiilities.
Expect to pay $5 for BTstack Keyboard when the software goes live. For more details, see Ringwald's Keyboard information page at his website. Ringwald is the same developer whose BTstack work brought iPhone-Wiimote integration into play recently.