If your blog is 2.1, it has the ability built right in! Previous 2.0.X versions require the techsailor plugin for exporting entries to the new XML format, but 2.1.X version have the ability right there.
Just register a blog and have it ready to import. The procedure would be as follows:
1) If you have WP 2.0.X, go to get the techsailor plugin and follow directions on that page. If 2.1.X, go to step 2
2) Go to the admin => manage => export of your current blog and export the content
3) Go to you new blog on wp.com admin => manage => import and import the file you exported from your other blog.
4) Download all your images from your upload folder of your current WP blog and unfortuntely you have to upload them to the new blog, 1 by 1.
There is an FAQ on this as well as documents in the wordpress.org support forums. Most are going the other way, but just reverse the process!
Hope that helps you think about it!