If there comes a time that you need to export subscribers from WordPress.Com follow these simple steps:
- login to http://www.wordpress.com
- select Dashboard : Site Stats
- locate Totals, Subscriptions & Shares metabox
- select Site Subscriptions :  active subscribers
- select either WordPress.com Subscribers or Email Subscribers
- select Download all as CSV
Also, I’ve found that the Subscribe2 WordPress plugin is the closest at emulating the WordPress.Com subscribe functionality and should be used to preserve the Subscribe (by email) and mailing list mechanics.