How to capture submit event using jQuery in an ASP.NET application?

Thanks, @Ken Browning and @russau for pointing me in the direction of hijacking __doPostBack.
I’ve seen a couple of different approaches to this:

1. Hard-code my own version of __doPostBack, and put it later on the page so that it overwrites the standard one.
2. Overload Render on the page and inject my own custom code into the existing __doPostBack.
3. Take advantage of Javascript’s functional nature and create a hook for adding functionality to __doPostBack.

The first two seem undesirable for a couple of reasons (for example, suppose in the future someone else needs to add their own functionality to __doPostBack) so I’ve gone with #3.

This addToPostBack function is a variation of a common pre-jQuery technique I used to use to add functions to window.onload, and it works well:

addToPostBack = function(func) {
    var old__doPostBack = __doPostBack;
    if (typeof __doPostBack != 'function') {
        __doPostBack = func;
    } else {
        __doPostBack = function(t, a) {
            if (func(t, a)) old__doPostBack(t, a);
        }
    }
};

$(document).ready(function() {
    alert("Document ready.");
    addToPostBack(function(t,a) {
        return confirm("Really?")
    });
});

Edit: Changed addToPostBack so that

1. It can take the same arguments as __doPostBack.
2. The function being added takes place before __doPostBack.
3. The function being added can return false to abort postback.