WatiN's DomContainer.WaitForComplete() method pauses test execution until the DOM has finished loading, but if your page has functions registered with JQuery's ready() function, you'll probably want to wait for those to finish executing before testing it. Here's a WatiN extension method which pauses test execution until that happens.

JQuery (as far as I can see) doesn't provide an event or other way of being notified of when it's finished running your ready() functions, so you have to get around it another way. Luckily, because ready() executes the functions it's given in the order they're registered, you can simply register another one to add a 'marker' div to the page, and tell WatiN to wait for that div to exist. Here's the code; I added the extension method to Browser rather than DomContainer (Browser derives from DomContainer) because it's the sort of thing you only execute once for each of the pages your test loads, so Browser seemed like a good place to put it.

public static void WaitForJQueryDocumentReadyFunctionsToComplete(this Browser browser)
{
    // Don't try this is JQuery isn't defined on the page:
    if (bool.Parse(browser.Eval("typeof $ == 'function'")))
    {
        const string jqueryCompleteId = "jquery-document-ready-functions-complete";
 
        // Register a ready() function which adds a marker div to the body:
        browser.Eval(
           @"$(document).ready(function() { " + 
                "$('body').append('<div id=""" + jqueryCompleteId + @""" />'); " + 
            "});");

        // Wait for the marker div to exist or make the test fail:
        browser.Div(Find.ById(jqueryCompleteId))
               .WaitUntilExistsOrFail(10, "JQuery document ready functions did not complete.");
    }
}

The code uses the Eval() method to send JavaScript to the browser to be executed; first to check that JQuery actually exists on the page, then to add the new ready() method. WaitUntilExistsOrFail() is another WatiN extension method I've written (I've ended up writing really quite a lot of them) which waits for the element on which it is invoked to exist, and uses Assert.Fail() to fail the test with the given message if it doesn't exist within the specified number of seconds. Here it is:

public static void WaitUntilExistsOrFail(this Element element, int timeoutInSeconds, string failureMessage)
{
    try
    {
        element.WaitUntilExists(timeoutInSeconds);
    }
    catch (WatinTimeoutException)
    {
        Assert.Fail(failureMessage);
    }
}