What this means is that you can
both sign in to WordPress.com using your existing OpenIDs (making WordPress.com a “consumer”) as well as use your WordPress.com URL (for example, http://factoryjoe.wordpress.com) as an OpenID elsewhere, making WordPress.com an iDP or “identity provider”.
The FAQ entry is pretty descriptive and I’d recommend you take a look at it. WordPress.com now joins a growing array of service providers offering support for this grassroots-driven authentication protocol.
No word on when OpenID will hit core of the WordPress project, but there are already two great efforts driven first by Alan Castonguay and more recently Will Norris — which point to a positive future between the two open source initiatives.