We are currently planning to create a 'preview' version for one of our asp.net mvc apps. Is it allowed to use a company like Microsoft's contoso for such a case (Branding etc.)?
There's no official statement by Microsoft but they do own the contoso.com domain. It's used in various products and most Virtual Machines used for hands on labs and such have a local domain named contoso.com along with @contoso.com users.
Fabrikam is another fictional company Microsoft uses (along with Northwind Traders, the source of the original Northwind access database). All of these domains are owned by Microsoft and redirect to Microsoft's main website.
I see no reason why you wouldn't use it yourself. I use it in all my demos and presentations but you might want to ping Microsoft by sending an email to email@example.com. Sometimes it's better to ask permission than to beg for forgiveness, especially with larger organizations.
I could not find anything regarding the Contoso database, but I did find Microsoft's AdventureWorks database in Codeplex, which is open source. If it were my app and I couldn't switch the preview to AdventureWorks, I would continue to use Contoso under the same license listed for Adventureworks.
But that's just me and I am in no way an attorney.
To further backup my assumption that using Contoso would probably be fine... Telerik, a Microsoft partner, uses the NorthWinds database on their web site as a sample database.