If one's ISP is using a transparent proxy the the IP address served up is the IP address of the proxy. This can be rectified by using the following blog http://www.thepcspy.com/read/getting_the_real_ip_of_your_users
Unfortunately there is also a possibility of multiple IPs being served up here if more than one proxy is passed through and there seems to be a debate as to whether the first or the last IP is the one you should use. see the following links for more info.
Hope this is useful for you