I had installed the AJAX extesions framework on my computer in order to assimilate it in my web application that I am working on right now. I added some AJAX controls such as <asp:ClientScript>, <asp:UpdatePanel> and more, and damn... I find that the markup intellisense no longer works for these controls, or for any controls nested within them.
This happens because my ClientScript control is defined in the master page, and when I tried to drag (create) some other AJAX control in the other aspx pages that inherits the master page, the intellisense was lost.
I found a great solution/trick in ScottGu's Blog, that says that in order to see that intellisense in the rest of aspx pages, we need to leave the master page designer open (isn't it little bit ackward?)
For some more information and tricks (in ScottGu's Blog) you can click here.