I have had some success with this. The following code does work but you only get an alert when the "CellID" changes which is not as useful as being able to read the current "CellID".
If you try to use
var criteria = new Object();
criteria.Entity = "Network";
criteria.Key = "CellId";
var result = so.ISysInfo.GetNotification(criteria, cellCallback);
alert ("Exception: " + e);
function cellCallback(transId, eventCode, result)
what you get is an exception with the text "Error: invalid transaction id".