I am in the process of going from an iMac and macbook air down to one machine.
Sometimes it is just a hassle having to keep files together on two machines.
Raw processing power is not a huge issue, but resolution is. As a researcher and writer, when doing work I have grown accustomed to being able to view multiple word documents and iphoto open to transcribe document images without having to switch back and forth.
At the moment I am splitting time between North America and Europe, and in the next couple of years will most likely be ending up on one side of the Atlantic.
I love the idea of the macbook and 24" display, but I am also intrigued about the idea of having the 24" display resolution everywhere, and not having to worry about either having to sell or move such a large item across an ocean.
I've never had a 17" notebook, but the battery, weight, and resolution seem really compelling.
I have buyers for both of my machines lined up, so the decision will happen soon, and am still torn 50/50, or 60/40 depending on the day.