One way to have a single signon for multiple portals is to use Windows Live or OpenID.

There is an article here
www.datasprings.com/Resources/ArticlesInformation/DotNetNukewWindowsLiveAuthentication.aspx

Windows Live Authentication support is one of the big new features with DNN 4.6.x, allowing you to enable an easy to remember login method for your users.   The great part about this addition to DNN is that it is an item that is easy to configure and requires no ongoing support to maintain. This article will take you through all steps necessary to setup your application with Windows Live and to actually enable the integrated authentication.

By Andrew Weitzen, Bronze Inc. (c) 2010

Bronze is the publisher of several online Internet journals including: InternetHandholding.com, DomainNames.gs, DotNetNuke.bz, Programmer.bz, Software.vg, WebHosting.vg