Is this application intended for a private network or the public Internet?
On the Internet, this approach will prove to be a problem on several fronts.
Technically, the growing use of NAT/PAT in homes with multipule computers means your chances of returning the wrong address (potentially a private address) from a client are increasing.
Ethically, the practice of picking up ANY information from the computer without full disclosure is similar to having a door-to-door salesman snoop in the closet while your getting him a drink of water. In the circles I run in, this practice would get your application black balled.
Picking up the IP Address of the client from their communications stream (as opposed to from their workstation) will make your application much more successful.