Here is a quick diagram that I put together for a 3-Tier application
the black boundary represents the trusted zone.
the blue lines represent the division of the 3 tiers
in my model app-logic is included in both the presentation and middle tiers so as to accommodate rich internet applications (where you will want at least some of the app-logic occurring in the browser)
for another graphic that I had posted about a while earlier – see: http://blog.aggregatedintelligence.com/2009/05/aspnet-3-tier-diagram-example.html