The adoguy is wondering if the browser is a "client tier". I think the web world is a flip-flopper.
To many the browser is a terminal. Pure and simple. Using it for anything outside of the basic capabilities is non-standard and should be avoided. In this worldview the browser is no tier. It is just a terminal, and granting it “tier“ status would mean granting 3270 mainframe terminals that same status - thus saying that the COBOL guys from 30 years ago did n-tier development. Go there if you want, but I'm skeptical.
To many others the browser is a programming platform. The “new rich client”, against which we can and should target rich, interactive programs. In this worldview the browser is a place to run code. It is not a terminal, it is a programming platform just like Windows, Linux or anything else. In this worldview you bet the browser can be a tier. Heck, you could probably write your UI and business layers into the browser tier if you really wanted to...
Honestly I thought this debate died with the dot-bomb. The debate was a growing concern back in 2000 or so, and then it rather died off when the excess money evaporated at the end of the Clinton era boom.
Though certainly some organizations have undertaken the massive pain and cost associated with targeting the browser as a programming platform (examples include Microsoft with Outlook Web Access, and as someone pointed out, Google with gmail)... But it is a losing battle for most of us, because there are no development tools.
But the only truly viable tool today (for developers who want productivity and reasonable development cost) is Flash. And with the recent advent of laszlo it looks like Flash is seriously on the move as a community effort. Now if only there was a laszlo equivelent that could be hosted in ASP.NET we'd be talking!!
So my take on it is this.
But if you are like most people, the browser is a terminal - a glorified, colorful mainframe terminal just like we've had for decades. In this worldview it can't be considered a tier because no meaningful code runs closer to the user than your web server.