filterObject.js

dojo.filterObject = function(obj, filterFn, context){
    var ret = {}; context = context || window;
    for(var i in obj){
        if(filterFn.call(context, obj[i], i, obj)){
            ret[i] = obj[i];
        }
    }
    return ret;
}

or... dojox.lang.functional.forIn

yah it's like forEach for objects, but is actually more like array.filter in that the return of the callback is used to determine if the item should be included in the copy.

sorry... the link was to filterIn but i wrote forIn - i meant filterIn which works as you say, i blame jetlag :P