You seem to want
http://www.bluevisionsoftware.com/We...msIntellisense
Use their generator tool (linked at end of article), and generate an xsd file for each of your control tagprefixes (for example, I have a user controls one and a server-side user controls one, for historic reasons I don't want to get into 'cos they'd show how new I was at ASP.NET ;) ).
Put those in Program Files\Microsoft Visual Studio .NET 2003\Common7\Packages\schemas\xml
In the head node of the documents that you want intellisense in (<body> for pages/webforms, but in .ascx controls, the first tag of the control), put xmlns:Var1="urn:Var2", where
Var1 is the tagprefix of controls (from <%@ Register TagPrefix="XrmHelp" ...%> statements)
Var2 is the namespace of the xsd, which apparently is usually a url because they're more likely to be unique.
I would love to know if it's possible to put these xmlns links in someplace that doesn't get output to the client-side page, but I don't know how to do that yet, or if it's possible at all. If it is, please someone let me know! ;)
{edit} Oh yeah - in an ideal world of course, someone will modify the generator tool so that it can take command-line arguments and thus be used in the build process to regenerate the xsd files upon recompilation so that changes are propagated to intellisense...