Does anyone know if it's possible to have another application in this case Salesforce.com Self Service Portal control authentication into the forums?

Here is a brief explanation from Salesforce about using Self Service Portal authentication as a SSO source for another web application.

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You can automatically login a user who is logged into the self service portal into an internal site by following the instructions below.

A use case example is:

1. User logs into Self Service Portal

2. User clicks link to other web application

3. Application can verify who the user is by being able to verify the session that they are currently logged in with self service.

you can call the getServerTimestamp API call with a valid CSS session id, so if that call returns, you know the sessionId is valid.

Customers can add a link like this on their homepage:

href="http://www.company.com/sso.jsp?sid={!User_Session_ID}&apiendpoint={!API_Partner_Server_URL_50}&userid={!User_ID}&doc
umentid=123456">

Then the sessionId can be used to verify the user is logged in (by making a successful call to getServerTimestamp), and the userId can be used to match them up on the company system. Note that any of the self service merge fields can be used: Username, email, custom fields, etc.

This requires an organization that has API access, and knowledge of a language such as Java, C#, VB.NET, Perl, or another language that supports web services. Information on how to call getServerTimestamp is in the API documentation.

Our OneCommunity API is available to customers with Business- and Enterprise-class plans, and it allows Eve to integrate with SSOs (or even act as the SSO).