You want portability as well as a large screen and would like to stay below $2500 for everything. What confuses me is why would drop that beast of a system for a much inferior version. It raises the eternal question:
What will you be doing on this computer?
If you plan on video editing, 3d work, or playing games stick with the beast. If the computer is primarily for 2d tasks in photoshop, iphoto, making dvd's, internet etc the entry level imac would be more than enough.
If you are considering a netbook, you probably don't need much power on the go, so I would rule out the powerbook idea as you are paying an incredible premium for such a small enclosure. I have the top imac and plan on getting an apple netbook. I use my desktop for video editing, maya, and games but still like being able to hang out on my sofa, or at a cafe, and browse the internet or write.