In Illinois, they are. I found this from the news feed down right. From NPR (you can listen to the story by clicking the link):
GPS to Warn Domestic Violence Victims
Audio for this story will be available at approx. 3:00 p.m. ET
Day to Day, August 8, 2008 · This week, the governor of Illinois signed legislation that will allow the state's judges to tag domestic violent offenders with GPS tracking devices. The abusers' victims will get monitors that warn them if their attacker is in the area. Alex Chadwick talks with Michael Bischof, who spearheaded the movement to pass the bill after his sister's death.
Sounds like a pretty good idea to me, particularly considering this freaking tragedy that happened in Portland just yesterday.