You may not need more than that for your apps. But if you have something that needs to communicate with a web service, a mainframe, some external device, you may add additional layers for each of those. Learn about Service Oriented Architecture (SOA) to see how it's done. A really good book is this one http://www.amazon.com/Service-Oriented-Architecture-SOA-Technology-Computing/dp/0131858580/ref=pd_bbs_sr_2?ie=UTF8&s=books&qid=1212163535&sr=8-2

>Maybe I'm just locked into an idea I have been familiar with for a while but I don't know why I would need more than a GUI layer, a data layer, and a business rules layer.