Themes can't be uploaded to WordPress.com. That's a feature of self-hosted blogs through WordPress.org.
http://en.support.wordpress.com/themes/adding-new-themes/
However, we offer over 200 themes, both free themes and premium themes created by leading theme companies. You can view all of our themes at http://theme.wordpress.com/
You can also post suggestions for new themes we might add on our public user forum:
http://en.forums.wordpress.com/topic/how-to-use-the-themes-forum