I have a application that exports HTML containing CSS to a wordpress blog. If I try that on my own blog that I installed on my own server (out-of-the-box) it works like a charm. When I do the same to my blog on wordpress.com it shows the stylesheet as text on the blog, it removed the 'style type="text/css"' '/style' tags.
Does this have a special reason? Did I forget to switch something on on my wordpress.com blog?