Though Microsoft's Web site appears to indicated [sic] otherwise, the company has acknowledged that, going forward, critical Internet Explorer security updates will only be available for Internet Explorer (IE) running on Windows XP. From a security perspective, the move officially strands users running IE on Windows 2000, Windows 98, Windows 98 SE, and Windows Millenium....
I'd say they will have to back off - as we all know IE is an integral part of an Operating System *wink wink*, and therefore refusing to support part of system equates to refusing to support the whole system, and this might breach legal contracts that they have with businesses regarding support of OSes.
Thing is, despite the whining on Slashdot and elsewhere, this is a complete non-story.
Microsoft has simply said that the specific improvements to security (such as the changes to restrict auto-donloading of ActiveX objects and such) which were offered in XP Service Pack 2 will not be backported to older Windows versions. This is not new.
Patches for vulnerabilities on IE6 on older versions will continue to be patched as before, in accordance with the usual MS support contracts and EOL limits.