WP.com offers a $99 guided transfer: http://en.support.wordpress.com/guided-transfer/
But if that doesn't meet your budget, tf you still have access to your .com site, you can do this yourself
- In the old blog, go to Tools → Export and download the XML (WXR) file to your computer.
- In the new blog, go to Tools → Import → WordPress (WordPress will prompt you to install the import tool at this point) and upload the file you downloaded in step 1. Make sure that you choose the option to Download & import file attachments. This option will be available after you upload the file.
That should get all your posts and content over.