A while back I read a post by Boris about how OpenID is not really easy to use (yet). He is completely right, and if Boris can’t use it, our moms definitely do not stand a chance.
I had a conversation about this with Ted Rheingold of Dogster, who was thinking of implementing OpenID for their users so people could user their Dogster logins to log in to affiliate third party sites.
A very important issue for him is that a lot of his users are not geeks and do not really want to get into the technological side of things. In most cases people will not be familiar yet with OpenID and you want to shield them from the complexities while still offering the benefits.
Do I have a OpenID?
When confronted with an OpenID login box, this is the first question that people —like Boris— are confronted with. What is this OpenID thing and do I have one or where do I get one?

Luckily more and more sites are offering hosted OpenID identities to their users. Wordpress.com does this for their blog owners and LiveJournal does this as well. Most people will probably prefer to use one of these hosted solutions offered by a third party site instead of hosting their OpenID themselves.
This way identities will be created until most people will have multiple OpenIDs. That still does not solve the problem of knowing that you have an OpenID and knowing what it is. I will propose a solution to this problem just after the next point.
URLs for what?
The whole concept that you use an URL to login —though I think it is quite elegant— will be difficult to explain to users, who already have trouble telling their login names and e-mail addresses apart. Adding another entity that you can use to login at sites, will only add to the confusion.
Signing in with e-mail addresses is firmly settled but it did take some time to get there. We may get to the same level with OpenID (and hopefully replace e-mail based logins altogether) some day but that is too distant currently. URLs are generally perceived as user unfriendly and normal users should not have to deal with them too much (yet).
Maybe i-names will be a solution to this sometime, but I don’t see it becoming mainstream any time soon.
Solve away the URLs
Taking both previous points together: most people will use a hosted OpenID solution and people do not want to type URLs, we can just abstract away the URLs completely.
When logging into an OpenID consuming site, that site can provide a selector with a couple of well known sites providing OpenIDs. This list of OpenID providers should be attuned to the target audience so they are familiar with these sites. With a fairly small list of providers, you can probably cover a large part of your user base.
I have made an example login box that works this way. It gives users the choice between several well known sites or the possibility to fill in your own OpenID. This is just a mockup which you can adjust in any way you like. You could expand the different login options or present them anyway you like. A site which already takes such an approach is the site for the band Rooney. You could also display the generated OpenID to the user at some point to get them accustomed to the OpenID they will be using.

Using that selector and a textbox users can pick a site they have an account on and fill in their username. The consuming site can then construct an OpenID URL from the given username and use that to log the user on. So taking my Wordpress.com username illustir it would construct my OpenID http://illustir.wordpress.com/ automatically (see the example).
What site are you taking me to?
The step where you leave the site you are logging into for another site can be a bit distressing for users. The approach that sites such as Wordpress.com take by having their own identity provider which looks and feels familiar dampens this transition a lot.
Large sites using OpenID should generally have their own provider so that they can control and attune the experience for logging users in.
Dogster’s use for OpenID
Suppose Dogster wanted their users to be able to log into third party sites using ther Dogster login credentials. This seems like it is exactly the kind of problem that OpenID is meant to solve. Especially in the case where the login is more a dependent syndication —a third party site affiliating with a bigger site— than that it is a general login (though nothing stops it from working that way as well).
So in the Dogster case they should start their own OpenID provider and OpenID enable all their accounts which are both relatively easy steps. Then, third party sites could use a Dogster login to log onto their site by simply becoming an OpenID consumer and by constructing the correct OpenID from the Dogster login.
The only problem with the Dogster case is that they use e-mail addresses as usernames and you would have to construct an URL with the e-mail in it. You probably would not want to spread e-mail addresses in that fashion.
This approach can be taken by any big site which wants to enable its users logging in elsewhere with the same credentials.
Update: I updated the example to be more clear and more educational about the actual OpenID that is being constructed.
Besides that a lot of people are missing the point. I am completely in favor of browser integration rich identity homepages and everything. Go out and build them already, but should/could/would are not going to help us right here right now. Given Livejournal+Wordpress+AOL almost everybody already has an OpenID but most of them do not know it yet. This —admittedly trivial example— is meant to fix that.