Are you re-blogging the content from your main site to a blog hosted on wordpress ? You might be jeopardizing your wordpress.com blog in that case.
Some people may want to do this as there are SEO implications involved and bloggers may benefit from doing this. But think for a minute, is there any use from this to any body at all ? Isn’t it a case of Spam Blogging ?
There might be genuine cases like you would have started blogging on a subdomain on worpdress.com and then want to shift to your own domain. When you do so, the best thing is to inform your readers that you have moved to a new site and give them the URL of the new site. I know you’ll loose a lot of SEO and traffic benefits which you would have built to the sub-domain hosted on wordpress, but there is no other better option that I know of.
If you plan to double post the same content on both the blogs, self hosted and wordpress hosted, your wordpress hosted blog can get suspended. This is not good for your main blog too as Search Engines will find the same content in more than one blog, and your blog might get penalized for duplicate content.
Please refer the Staff’s Response at http://en.forums.wordpress.com/topic/automatic-posting-to-wordpresscom for more clarity on this.