why would one buy the arduino bluetooth board ($162 cnd) when one could buy the duemilanove usb ($30 cnd) + (bluetooth dongle ($20))?
Am I missing something?
You can actually pick up bluetooth dongles for $4 now, but unfortunately it's not as cheap or simple as that.
USB devices require a USB host controller to operate, which is what is inside of your computer to allow it to connect to bluetooth, usb hard drives etc.
The arduino has a simple USB connection to allow you to connect to a host controller and transfer data to the board. The arduino its self isn't a USB host controller, so you can't just connect a USB device to it and expect to be able to use that device.
Another problem is that a lot of USB devices, such as the bluetooth adaptor, use the host computer for most of the processing (through the driver). Even if the Arduino had a USB host controller, you would need to write all the code to handle the bluetooth protocol, which wouldn't leave much available for the projects purpose.
The BT Arduino has extra components to handle the bluetooth protocol etc, and so costs a lot more than the standard arduino.