New Post: Problem using UpdateListItems

I've used SPServices to query a list, create new list items, and grab attachments from an Announcement list but this is the first time I've used it to update a list.

jQuery: 1.7
SPServices: 0.7.1a
Permissions: double-checked

I've banged my head against this for a couple days and can. not. see. the problem. All I need to do is update the "Project_x0020_Stage" LOOKUP column. For the value, I also tried using a full lookup column value ("8;#Beer").

$().SPServices( {
    operation: "UpdateListItems",
    async: false,
    batchCmd: "Update",
    listName: "Project Milstones",
    ID: 63,
    valuepairs: [["Project_x0020_Stage", "8"]],
    completefunc: function(xData, Status) {
        console.log( $( xData.responseXML ) );
        alert("project updated");
    }
});

The completefunc executes every time. That said, I'm not going to pretend I truly understand what's going on under the hood.

Thanks for your time.