You might be interested to know . . . WB did support vector stuff in alpha at one point. The idea was partially to improve performance. It never really worked out. Skinners (particularly of the artistic sort) were used to making images in paint programs and the restrictions of the vector format were too great. WB's performance with image moving also improved to the point where it might actually be slower to use vectors (remember, it's a lot better to pre-calculate stuff than to do it every time). So, skins nowadays are made using raster images, even if those images are made in vector packages and exported.
It may be that Longhorn will bring change, but remember that XP will be around for a long, long time - it is perhaps the first home-user OS that is stable enough to last the test of time, and considering Avalon will be back-ported, it may not be necessary for most people to move onto Longhorn. I personally think people who wait for vector skins to become popular will be waiting a long time.
