Here is how I would approach it.

First, each user can be assigned to a security role, and you could define a security role for each company or client.  The trick would then be to automatically assign the roles, based on the profile.  You can do this most easily with a module like DataSprings' DYnamic Registration module.

Second, having assigned each user to a security role, you can use that to determine what information to show to a specific logged in user. 

• You can put conditional coding in your site skin.  Look at the DotNetNuke support forums a snapsis.com for some great examples.
• You can make entire pages or just modules on a page visible or invisible to certain security roles.  So, you could have three versions of a particular text/html module, with one being for each of 3 security roles.
• I believe that the new Tokens in the Text/HTML module include some conditional tokens.  You can use them within the Text/HTML module to show particular blocks of stuff to specific roles. 
• Token replacement is slowly migrating to other modules.  They are in UDT (where the first were introduced) and may now be available in other modules.  I haven't been keeping up, but I believe that the plan is to provide replaceable tokens for lots of the core modules, whereever that is appropriate.

As I said, that would be my starting point.  I hope that other also have some ideas that they can contribute.

Hmmm, I've searched high and low for info on "conditional coding" for DNN skins but have come up empty.  It seems that skin authors try to go with pure CSS skins.  The free skins I've downloaded contain no C#/VB code.  As such, I'm spinning my wheels.

Will someone please point my Jethro noggin to a resource that explains conditional coding for DNN skins?  Again, I'm looking to display company-branded logos in each page header based on the user's security role (or user profile data, such as company_id).

Many thanks!

Troy