Edit: This still works for VS2005 Beta2
Since Beta 2 is still a month off or so, I think this could still be relevant :-)
This blog tells you how to enable intellisense for NAnt .build files in VS2003.
This blog does the same for Wix .wxs files.
Both should be slightly altered to work with VS2005 beta1.
1) copy the schema files (.xsd) to the following folder: C:\Program Files\Microsoft Visual Studio 8\Xml\Schemas
2) the registry key with which Wix (.wxs) and NAnt (.build) files can be registered as being xml is now:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\8.0\Editors\{412B8852-4F21-413B-9B47-0C9751D3EBFB}\Extensions]"wxs"=dword:00000029
(replace the "wxs" by "build" for NAnt)
Note the subtle difference: dword value is 0x29 and not 0x28, which is ".htm" in vs2005b1, and the guid is different.