I would argue that the top level navigation button needs to be linked BUT have it go to a page that shows all your products or product groups if there are a larger number of items.
If your product page was to go automatically to your first product, and the user does not wait long enough to see the menu drop down they will think you only have one product.
Ex. Simple Menu
Products = (Product A, Product B, Product C, Product D)
Product A
Product B
Product C
Product D
Ex. Group Menu
Products = (Product Group A, Product Group B, Product Group C)
Product Group A = (A1, A2)
->A1
->A2
Product Group B= (B1, B2)
->B1
->B2
Product Group C= (C1, C2)
->C1
->C2
Example Site:
http://www.thevelveteenrabbit.ca/