there are various approaches possible for integration depending on concrete ncessicities of the integration:

• A custom membership provider for DotNetNuke ca be used, so DNN directly reads account information from Siebel
• Another approach would be to replicate necessary user information after each change from Siebel to DNN
• possible techniques for data exchange include direct database access, common business objects and web services

I have a bit of experience with integrating Siebel (though not with DNN) and a bit of experience building and integrating DNN apps.

The first question you need to answer is are you looking for real time integration or batch integration with Siebel.

If you want real time integration you'll probably need to build a custom membership provider. That custom provider will integrate in one of 2 ways. You can either use Siebel Integration Objects which are EAI xml messages sent back and forth. This is Siebels most recommended way but also requires the most Siebel configuration and expertise. The other way to build this is to do all reading through Siebels base database tables and all writing through Siebels business object layer using DotNet and COM interop. You could do all reading and writing through the Business Object layer but in my experience that can be very slow. This is why I prefer to read directly from the database and write through the object layer. What you cannot do is write directly to the database. Siebel does not support base table updates and you will invalidate Siebel support if you build applications that do that.

If you can get away with batch updating Siebel has something called EIM which allows you to load data into staging tables for importing and exporting data. You could write stored procedures to import/export on these EIM tables on a nightly basis. The batch update requires a bit less effort involved in the integration.