Contrid – thanks for the tip about the height and overflow. I never noticed that there before but the box seems to adjust as I type text. Did you copy the text of your comment into the comment area?
Anyway, are you talking about a newly generated page or an already cached page? If it’s an already cached page you’ll see from wp-cache-phase1.php that wpsc_cachedata is called early on so the shortcode won’t have run. Your filter function will have to generate the dynamic data.