Rachel Lucas has a post up about why all cats should be indoor cats. Admittedly the language is a little colorful -- but she's absolutely right. In fact, the only way your cat should be an outdoor cat is if you live in the country and it is a working animal such as a barn cat... OR... if you do something like this.
There are any number of cat fencing options:
Check it out.
Please -- if you love your cat (and you wouldn't be here if you didn't), keep 'em indoors or fence 'em in.