GolfScript (4343 42 chars)

' /\ /__\ '4/){.+\.{2$[2$.@\++]*}%\{.+}%+\}3*;n*

Output:

               /\               
              /__\              
             /\  /\             
            /__\/__\            
           /\      /\           
          /__\    /__\          
         /\  /\  /\  /\         
        /__\/__\/__\/__\        
       /\              /\       
      /__\            /__\      
     /\  /\          /\  /\     
    /__\/__\        /__\/__\    
   /\      /\      /\      /\   
  /__\    /__\    /__\    /__\  
 /\  /\  /\  /\  /\  /\  /\  /\ 
/__\/__\/__\/__\/__\/__\/__\/__\

Change the "3" to a larger number for a larger triangle.