Awwww yeahhhhhhhhhh.  I got my intellisense back that I've been lamenting about.   

Here's how:

1) If you're using MasterPages, don't @register user controls at the top of your page.

2) If you're using MasterPages, don't add user controls inside the

<Controls> sub tag of the  <Pages> tag in web.config:

<pages masterPageFile="~/extranet.master">
<controls>

<add tagPrefix="uc1" tagName="mainheader" src="/inc/mainheader.ascx"/>  <-- NO

3) DO specify your master page in the pages tag of web.config

4) Use placeholders for your controls: 

<asp:PlaceHolder ID="mainheaderplaceholder" runat=server/>

and load up your control with LoadControl either in Page_Load or Page_init:

Protected Sub page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
mainheaderplaceholder.Controls.Add(LoadControl("/inc/mainheader.ascx"))
End Sub

You have no idea how sweet it is to see that popup in my code-behind page when I press period.