How to have a javascript callback executed after an update panel postback?

Instead of putting your jQuery code inside of $(document).ready(), put it inside

function pageLoad(sender, args) { 
    ... 
}

pageLoad is executed after every postback, synchronous or asynchronous. pageLoad is a reserved function name in ASP.NET AJAX that is for this purpose. $(document).ready() on the other hand, is executed only once, when the DOM is initially ready/loaded.

See this Overview of ASP.NET AJAX client lifecycle events