Knowing someone's OpenID identifier has mostly the same security implications as knowing their login. The only difference is that an OpenID identifier is a url that points to some server, so knowing it would theoretically allow a malicious user to attack the identity endpoint (i.e. that server) - but that's not a security issue for your site.
Publishing it should be mostly safe, but whether it's a good idea is another matter. A human readable string (for example, a pseudonym) might be a better choice for a user identifier.
That said, some sites consider their users' logins a secret - most don't, but that's a choice you have to make yourself.