The Center for Disease Control has an annual rate of known rabies infections in (unvaccinated) cats at about 300 cases per year. Most of the rabies incidents are wildlife related. Here is a link which explains more about this. The last documented cat to human transmission of rabies in the US was 35 years ago:
http://www.alleycat.org/page.aspx?pid=686
There have been 21 cases of humans contracting rabies in the US in the last 10 years = while it is possibly that your outside cat may encounter a rabid animal, statistically she has a low chance of being infected, and an even lower chance of transmitting this to a human.
