MSXML4 will be kill bit-ed in IE
The Microsoft XML team has announced MSXML4 will be kill bit-ed in IE in the October - December 2007 timeframe.
The team is also *strongly recommending* developers who use MSXML to program with MSXML6 and upgrade apps using older versions to MSXML6.
Good background reading on the topic is this October '06 post by Adam Wiener explaining the versioning history of MSXML and best practices.