When talking to customers about WebSphere Portal architecture and solution design, I get this a lot: "So I have this existing Web application. How do I get it into Portal?" And the typical developer in me answers with "Well, it depends..."
In fact, it does depend. There are several application integration techniques for WebSphere Portal, ranging from low-fidelity controls like IFRAMEs all the way to the latest and greatest in web services for remote portlet (WSRP) technologies. I have cataloged almost all of the integration techniques in my Administration and Performance blog.
But isn't usually enough to know what your choices are. You also need to know how to make the right choice. There is something of a decision tree that I employ in my conversations, to help narrow down the choices. I took a stab at documenting a general purpose decision tree for application integration in my other blog as well.
I hope you find this useful. If you have other ideas or application techniques to share, let me know, and I'll happily add them to the list.