While official support ends in mid-July, Microsoft looked at fixing MS06-015 and said screw it:
Specifically, after extensive investigation, we’ve found that it’s not feasible to make the extensive changes necessary to Windows Explorer on these older versions of Windows to eliminate the vulnerability.
This is because during the development of Windows 2000, we made significant enhancements to the underlying architecture of Windows Explorer. The Windows Explorer architecture on these older versions of Windows is much less robust than the more recent Windows architectures.
Due to these fundamental differences, these changes would require reengineering a significant amount of a critical core component of the operating system. After such a reengineering effort, there would be no assurance that applications designed to run on these platforms would continue to operate on the updated system.
We do strongly recommend that customers still using Microsoft Windows 98, Microsoft Windows 98 Second Edition (SE), and Microsoft Windows Millennium Edition (ME) protect those systems by placing them behind a perimeter firewall which filters traffic on TCP Port 139 which will block attacks attempting to exploit this vulnerability. This is discussed in the "Workarounds" section of the vulnerability.
Well, this may elicit howls of protest from some, but I personally don’t blame them much. Low-level Win 98 development is a horrible, ghastly endeavor, and given the challenges they were faced with (like making apps continue to be compatible), I think they made the most logical decision.