I've used this script before. It sets both the number and short description if they are there. You'll need to use this code in a new UI script record under 'System UI -> UI scripts'.
addLoadEvent(setTitle);
function setTitle(){
try{
var num = g_form.getValue("number");
var sd = g_form.getValue("short_description");
}catch(e){
//alert('Error getting title info.');
}
if(num && sd){
top.document.title = num + ' - ' + sd;
}
else{
top.document.title='Service-now.com - IT Service Management Suite';
}
}
