We're developing an ASP .NET application and want to include a "support forums" feature. Any suggestions?

Lots. In fact, Wikipedia maintains a comparative list of internet forum solutions for ASP .NET here. One to consider is FuseTalk:

• Feature rich forum software, including
  
  • flat or threaded views
    
  • user-selectable themes
    
  • image attachment capability
    
  • calendar
    
  • WYSIWYG editor
  
  
• Ability to convert topics/threads into a knowledge base item, progressively building a knowledge base
  
• Simple integration, particularly for single sign-on
  
• Localized for translation in any language
  
• Written in ASP .NET so that your development team doesn't need to know another programming language to support it
  
• "Basic" license reasonably priced ($700)

To host the fusetalk solution however, your application will need to also host a MS SQL server.