JQuery :: Cannot Return Value After Ajax Call

I'm currently using the dialog functionality (modal form) from UI library to submit data to the database. All the validation checks have been running ok until one of the validation checks requires a ajax call to check if a username exist in the database. I'm fairly new to both javascript and jquery so this could be a fairly basic blunder. Initially, i thought it was an synchronicity problem, but I changed the $.ajax async option to true but still no joy, so maybe it something to do with scope etc?

Here's the code:
function checkIfUsername(){
$.ajax({
type: "POST",
url: ""+CI_ROOT+"index.php/admin/check_if_username",
data: ({username: username.val()}),
async: false,
dataType: "json",
success: function(data){
returnUsernameBool(username, data);
}});
}function returnUsernameBool(o, data){
if(data.bool == true){
o.addClass('ui-state-error');
updateTips("Your username must be unique");
alert('false'); //this works
return false; //still can't return bool
}else{
alert('true'); //this works
return true; //still can't return bool
}}
bValid = bValid && checkIfUsername(username);
alert(bValid); //still gives undefined