A while back, I wrote for the Times about the possible effects of the ending of support for Windows 98, and suggested that sooner rather than later a big vulnerability would come along for which there would be no fix forthcoming.
Well, MS06-051 may well be it. As the metasploit blog points out, any null pointer dereference or stack overflow is now exploitable. And Microsoft's bulletin clearly states that there won't be any fixes for Win98, or for Win2K/NT 4 for ordinary mortals.
There are so many ways of causing one of these that it's easy to imagine that, within a few weeks, someone will package up some code which exploits this through IE, or a browser plugin, or Active X or any number of other vectors. A couple of months after that, it'll be part of the default payload for those "multiple exploit" pages which try ten different things against your browser to see if one of them works. A few XSS-injected redirects on popular sites, and surfing the web with 98 becomes like playing Russian Roulette.