I have been tinkering with different JavaScript Intellisense scenarios in Visual Studio 2008.  I just stumbled onto a feature I did not know existed thanks to an internal discussion.  First, if you use a ScriptManager in your page to reference other JavaScript files and service references like so:

MyPage.aspx:

...

<asp:ScriptManager ID="ScriptManager1" runat="server">
                <Scripts>    
                    <asp:ScriptReference Name="PreviewScript.js"
                        Assembly="Microsoft.Web.Preview" />
                    <asp:ScriptReference Path="MyPage.aspx.js" />
                </Scripts>
                <Services>
                    <asp:ServiceReference Path="OfficesService.svc" />
                </Services>
            </asp:ScriptManager>

...

Next, if you like to put all of your JavaScript in a separate "code beside" like I do, then all you have to do is reference MyPage.aspx in MyPage.aspx.js like so:

/// <reference path="MyPage.aspx" />

Once you have done this, you will get JavaScript Intellisense in MyPage.aspx.js for everything that was referenced in your ScriptManager.  Now if you add a reference to the ScriptManager, you automatically get JavaScript Intellisense in your "code beside" .js file.  Before I found this feature, I would add duplicate "/// <reference/>" entries in my MyPage.aspx.js file corresponding to the entries in my ScriptManager.  It was a maintenance hassle as references changed.  Problem solved:). 

-Marc