Jack Van Hoof has a different view than I have on the difference between Tiers and Layers. I am not sure I agree with his view, but it still provides an interesting read. I think  the main difference between our respective views is that Jack takes a look form the enterprise-architecture angle which gives him layers like

• Technical infrastructure - OS, directory Services etc.
• Application infrastructure - Apps, Portals, DBMSs
• Application Landscape - SOA, EDA
• Bushiness Processes - BPM

Jack uses the term tier for layers within the same level of abstraction. for instance he gives the following examples:

"E.g. the layer of business services may be arranged in the tiers: front-office, mid-office and back-office. At the next lower layer, the application layer, services may be arranged in the tiers: UI, business logic and data persistency. The interaction of services between two tiers may be bidirectional (but may also be constrained to unidirectional). "

The perspective I have (or at least try to maintain in this blog) is the solution/product line architecture - basically living within Jack's application layers. So in my view I want to know and differentiate between the difference of having a UI and business logic live on the same machine vs. having them distributed in the world. So I guess in the end both perspectives need to have their place and the problem is, like many other times,is  overloaded terms