Elledan,
I read through the Microsoft tech bulletin and only in XP is the UPnP service installed and running BY DEFAULT. In ME one has to install it seperately, and in 98/98SE the user would have had to install the XP Internet connection sharing client.
Here is the MS assesment by OS:
What operating systems support UPnP?
* Neither Windows 98 nor Windows 98SE include a native UPnP capability. It can only be added by installing the Internet Connection Sharing client provided in Windows XP.
* Windows ME includes a native UPnP capability, but it is neither installed nor running by default.
* Neither Windows NT 4.0 nor Windows 2000 support UPnP.
* Windows XP includes a native UPnP capability. It is installed and running by default.
Link to full bulletin
In a sense you are correct, ME users and 98/98SE MIGHT be at risk (ME more probable than 98) but the real critical vulnerability is to the unsuspecting XP user who has a default install.