Twitter Bootstrap Typeahead wrapper that uses objects instead of strings (lite version) -- see example http://jsfiddle.net/VjVMf/.

index.html

<input type="text" data-bind="value: customerAC.query, jqueryui: { widget: 'typeahead', options: { source: customerAC.source } }" placeholder="Start typing customer name..." />
<br />
<strong data-bind="text: selectedCustomerId"></strong>

knockout_autocomplete.js

function KnockoutAutocomplete(baseUrl, observableValue, filterCondition) {
    var self = this;
    self.selectedItem = ko.observable(null);
    self._itemsMap = {};
    self.updateQuery = ko.observable(false);
    var observe = typeof observableValue === 'function';

    if (!baseUrl.match(/\/$/)) baseUrl += "/";
    self.search = function(query, key, process) {

        ko.utils.get(baseUrl + 'autocomplete', { query: query, key : key }, function (data) {

            // filter
            var filteredItems = data;
            if (typeof filterCondition === 'function') {
                filteredItems = [];
                $.each(data, function (i, item) {
                    if (filterCondition(item))
                        filteredItems.push(item);
                });
            }

            process(filteredItems);
        });
    };

    self.query = ko.computed({
        read: function () {
            self.updateQuery(); // force update in case if self.selectedItem wasn't changed
            return self.selectedItem() ? self.selectedItem().Text : null;
        },
        write: function (title) {
            var item = title && (!filterCondition || filterCondition(title))
                ? self._itemsMap[title.toLowerCase()]
                : null;

            self.selectedItem(item);
            self.updateQuery(!self.updateQuery());
            
            if (observe) {
                var newValue = item ? item.Value : null;
                observableValue(newValue);
            }
        }
    });

    self.source = function (query, process) {

        self.search(query, null, function (items) {
            // map
            var titles = [];
            $.each(items, function (i, item) {
                self._itemsMap[item.Text.toLowerCase()] = item;
                titles.push(item.Text);
            });

            // display
            process(titles);
        });
    };

    self.selectByValue = function (val) {

        if (!val) {
            self.selectedItem(null);
            return;
        }
        
        // try find within saved items list
        for (var title in self._itemsMap) {
            var item = self._itemsMap[title];
            if (item && item.Value == val) {
                self.selectedItem(item);
                return;
            }
        }
        
        // else retrieve from server
        self.search(null, val, function (items) {
            if (items.length > 0) {
                var itemForSelection = items[0];
                self._itemsMap[itemForSelection.Text.toLowerCase()] = itemForSelection;
                self.selectedItem(itemForSelection);
            }
        });
    };

    if (observe) {
        observableValue.subscribe(function(newValue) {
            self.selectByValue(newValue);
        });
        self.selectByValue(observableValue());
    }
}

model.js

var model = {
  selectedCustomerId: ko.observable()
};
model.customerAC = new KnockoutAutocomplete('/customers/', model.selectedCustomerId);

$(document).ready(function() {
    ko.applyBindings(model);
});