When using master pages in ASP.NET, visual studio 2005 often gets confused and loses its ability to display intellisense in your content pages. IE: Within your asp:Content tags, if you type asp: you won't get any intellisense for the standard asp controls.
There's a simple work around for this and that's to ensure you have your master page file open in a tab in the editor. If it's open, you'll get the intellisense back in your content page.
I'm sure this will be fixed in a later release, but until then, the above should suffice.