http://lists.oasis-open.org/archives/imi-comment/201004/msg00001.html

Section 2.6.1 discusses the use of the Address attribute.
"While moderately effective, this practice often proves impractical for services offered to large user populations, many of whom are likely to encounter proxies and network configurations that result in inability to satisfy the restriction"

Just wanted to add as an observation that this is also impractical in situations where a selector relies on a cloud-based service for talking to the STS.
E.g. the iPhone I-Card Selector works like that, therefore the IP address seen by the STS is always different from the IP of the selector.