Interesting, I've just tried to go into mine and rename my handsfree, no luck at all. Would be interested to know if this is possible also. I know with other phones I've had that this has been an option.
4:22 am 27/08/11
I've just had the same problem and found this is the only available solution so far:
Your iOS device needs to be jbroken so you can access its local files! I've used Cyberduck to connect to it, iPhone Explorer or any other should also work just fine… please google if you don't know how to access the files on your iOS device!
Go to the following folder on your iPhone: "var/mobile/Library/Preferences" and copy the file "com.apple.MobileBluetooth.devices.plist" to your computer.
Open the file with Property List Editor, look for your specific device by opening the BT MAC address(es) and set a new device name under "Name" (not DefaultName).
Save the file and copy it back onto your iPhone overwriting the existing one. You might want to check the correct permissions (600) and then you NEED to reboot your iPhone!