First grab rohos and install.
Run rohos and hit "Choose USB key device"

Select your iPhone and "Choose USB"

Type your password

Turn the Key on

Great! Your iPhone / iPod Touch is now a usb security Key. To lock your computer unplug your iPhone / iPod Touch. To unlock plug it in. Of course you can also make flash drives etc into a security key as well but what's the fun in that?
The downsides. The software is a free trial you have to buy it or abandon it after 15 days. The rohos software on windows doesn't allow selection of the iPhone / iPod Touch as a USB device. You will get ridiculed by normal people if you use this in public.
thanks pelchat