Even if there were such a thing as a Bluetooth ➔ WiFi router (I've never seen or heard of one), and you have tethering enabled on your phone and cellular plan, I don't believe it would work, depending on the Bluetooth profile supported by the router. The only type of Bluetooth file transfer profile supported by iPhones is a direct tethering connection to the end device. Unless the router appears that way to the phone, it won't work.
If you have an iPhone 4, WiFi hotspot will be supported directly from the phone with the 4.3 update.
Message was edited by: modular747