Just looking at the industry right now, the majority of Stardock's customers as far as customization products would have to be individual consumers, and residential computer users have a tendency to, especially in times of recession, make the old work for them instead of moving on to the latest and greatest thing, whatever it is.
Many people are going to continue using XP for years. Mainstream support for it ended only recently, and even taking that out of consideration, there are still people, although the numbers are shrinking, that are still using Windows 98 and ME. Many businesses, and individuals who use their computers for both business and personal use, will likely be sticking with XP for some time in the interests of maintaining compatibility, despite the availability of XP Mode. A note to anyone considering migration with interest in this feature - make sure you select the right version of the OS. Windows 7 Professional and Ultimate are the only versions with access to XP Mode. Most systems purchased at retail will include Home Premium, and will not have access to that feature without upgrading to one of the other versions. There are other ways, but those aren't part of this discussion...
Personally, I've upgraded everything to Windows 7 Professional, as personally, I don't have a need for the extra features included with the Ultimate version. (Our MAPS subscription gave us early access to the RTM.) But for true compatibility, especially in terms of hardware, I am keeping one system running XP.
The fact is that XP will still see a good user base for at least several more years, especially in terms of netbook owners, since most of these that are sold, at least here in the States, include XP Home as the preinstalled operating system. There is already an extensive library of skins available for XP, but to keep the product going strong, I see no reason yet to cease production of skins for XP. Everyone loves shiny new toys.
As more people migrate to Windows 7, there will be a decrease in demand for skins for older versions of the OS, and eventually Stardock will likely drop support in later versions of the software, forcing our hands if nothing else.
The bottom line, unless you're under commission or something, do what you like to do. If you like to skin XP, do it. If you want to develop new eye candy (or something more minimalistic) for 7, then feel free. I, as well other users, will be appreciative of what you create, regardless of which direction you take.
