OpenID delegation is quite possible with wordpress.com.
If you run a WordPress blog on your own site, for example, and want to use WordPress.com as your Open ID server, you would just add these lines to your other server's webpage (in the <head> section):
<link rel="openid.server" href="http://YOURNAME.wordpress.com/?openidserver=1" />
<link rel="openid.delegate" href="http://YOURNAME.wordpress.com/" />
This delegates the open id functionality for your domain over to WordPress.com.
The "openid.server" tells the OpenID requesting system where to go talk to to get OpenID info.
The "openid.delegate" tells the OpenID requesting system who you actually are on that other system.
This works absolutely correctly as near as I can tell.
However, wordpress.com's OpenID server doesn't actually appear to work at all right now. I've tried dozens of OpenID enabled consumers, and WordPress.com never actually prompts me to login, regardless of what I do. The OpenID tester fails to function with WordPress.com for anything beyond the initial setup sequence. I do not know why.
Official tester is here: http://www.openidenabled.com/resources/openid-test/diagnose-server/