Hey Toastie,
Just signed up as i was having the same problem as you but couldn't find the answer anywhere.
After playing around a bit, i got the phone to be recognised by win 7.
Try this:
Plug your iphone into your computer
Go to device manager,
You will see an unidentified MTP USB device, right click on it and select uninstall.
When that is finished click on ACTION at the top of device manager and then click on Scan for hardware changes.
This will reenumerate your device and windows will install the needed driver.
Make sure you have iTunes installed before you do this.
Hope this helps.