I'm just getting myself into the same position.  I have just started development on our companies website in DNN and while I am planning on continuing to use Sharepoint for our employees I will have some users that will need to log into both.  If you find a good path to follow for a single sign on, please post.

 

After some more testing and painful experimentation, it has become clear that (1) using Active Directory and Integrated Windows Authentication for DNN and SharePoint is impossible, because DNN can't handle true integrated Windows Authentication. It just checks credentials, then uses Forms Authentication to run. (2) Creating separate sites with mixed authentication doesn't work, because DNN is a single web application with a single web.config and single authentication method. (3) DNN must use its own SQL security tables for a completely unknown reason (they look like standard SQLMembershipProvider tables, but there are a few things missing). So, after a week and a half of painful revelations, I've decided to share our plan. If anyone else needs this solution, you can try this as well. If I'm successful, I'll post instructions on how to do this for real.

THE PLAN:

1) Existing SharePoint intranet. Use integrated windows authentication for a seamless experiece for staff. Build a module (or buy the SW.AutoLogon) to allow staff to be automatically logged into DNN. Voila, single sign-on for internal staff. Of course, its one-way only, but this shouldn't be a real problem.

2) New public site on DNN. Nothing special here, just the plain vanilla DNN public site.

3) New members-only site on DNN. Use Forms Authentication and the DNN database for member credentials.

4.a.) New extranet for members/staff on DNN. The combination of #1 and #3 should give single sign-on for everyone involved.
--OR--
4.b.) Extranet on SharePoint. This may or may not be a problem. First, SharePoint and DNN have to be on the same domain and both have to run in Forms Authentication for member sites, although they could be on different sub-domains and still share cookies for Forms Authentication (with some customization). Next, member user information will have to be synchronized between the DNN and SP databases, or SharePoint would have to somehow use the DNN "aspnet_" user tables. I've had limited success with some testing, but it hasn't totally worked yet (I'll let you know).

Eventually, I'll get SSO for everyone, and I'll share the results. I'd like anyone else who is attempting this to also share ideas. Thanks!