You could also try something like the following. This is what I use on my site to get users for populating jQuery UI's Autocomplete dropdown
$().SPServices({
operation: "GetUserCollectionFromSite",
async: true,
completefunc: function (xData) {
var users = [];
$(xData.responseXML).find('User').each(function () {
var item = {},
o = $(this);
item.value = o.attr('ID');
item.label = o.attr('Name');
users.push(item);
});
UT.USERS = users; // This is a global var that I use
}
});