Today AOL relaunched the OpenAIM initiative. One of the key parts of this effort is supporting authentication for clients (whether those clients are native code or FLASH/AIR/Silverlight/etc). To enable this we added a 'clientLogin' API to our OpenAuth suite of APIs.
This support is to enable a user experience that comfortable to AOL's existing members. If a user installs a client application, it is expected that any authentication that is needed is done through the client. To secure this method we use the password and a session key (transfered via SSL) to sign all requests from the client application. This ensures that all valid requests are coming from a client where the user entered their password. In addition, all API calls are monitored for abuse.
For the signing mechanism we used the OAuth signature base string method. However, given that we already had parameter names (in existing APIs) that map to the OAuth parameter names, we used the existing parameter names in favor of the OAuth names. Otherwise, the logic is the same.
I realize that from a pure privacy and security perspective, the OAuth flow of "popping" a browser and having the user only enter their credentials at the "owning" IdP is better. However, for many of our customers this is an unexpected user experience. clientLogin enables the desired user experience.