Interesting thought...technically do-able, yes. You are correct in that finding the LCD for "low-cost" would be the toughest part. Really portable? Probably not. Might be an interesting toy, but the weight of the power supply from the "desktop" market would itself outweigh most notebooks.
Here's a different angle to your thought...could an all-in-one ATX motherboard drive a notebook shell? If one purchased a $300 close-out laptop, could an ATX motherboard and the notebook shell be married together? Ignore the size issue for a moment (I'm not asking if an ATX motherboard could go inside the notebook shell), but focus on the integration of the technologies.
Could the video adapter on the ATX motherboard drive the LCD panel?
Could the IDE controller on the motherboard drive the notebook harddrive? How about the second notebook (hot-swappable) IDE component like a CD, DVD, ZIP, CDRW, 2nd drive?
Could the motherboard IR header drive the notebook IR receivers & transmitters?
Could the motherboard port controllers (USB, serial, parallel) drive the device recepticles in the notebook shell?
Could the motherboard keyboard controller work with the notebook keyboard?
Could the motherboard mouse controller work with all the various mouse devices (touch pad, j-stick, track ball, etc.)on the notebook shell?
What controller on the motherboard would drive/work with the PCMCIA/PC-Card slots on the notebook?
What might be a real interesting (new??) market for current desktop motherboard makers (ASUS, ABIT, etc.) might be a generic notebook motherboard replacement. What I don't know is how tightly integrated (besides the connectors) the notebook hardware devices are to the motherboard.
My gut reaction.....the biggest problem with a generic notebook replacement motherboard is that none of the notebooks share a common system-board footprint (in size or shape), and none of the hardware components share the same connector.
Notebook manufactures have worked themselves into a nice, but expensive, niche. Everything is very unique to each model of the notebook (even within the same manufacturer) making it difficult for the general consumer to upgrade pieces (ever wish your notebook display adapter could run at a higher resolution than 800x600?). But it is also means that the manufacturer has to support each unique model.
Which is a better business decision? Have many models which are unique in design forcing the consumer to completely toss their old machine when they upgrade? (Consumers will take longer to upgrade since they want to make their investment last as long as possible. Plus, there is no garauntee that the consumer will upgrade to the next version of the same manufacturer's model!)
Or would it be better for all models to share the same design of components which are modular enough to be interchangeable? Consumers could upgrade compents of their notebook when new technology comes out, and replace the "external facing pieces" (LCD screen, 9-pin serial port, keyboard, etc.) when they wear out. (How often does one need to replace/upgrade the keyboard? How often does one wish they could replace/upgrade the CPU or video adapter?) As a laptop manufacturer this process will insure that the consumer will stay with your products.
I think that the best advocacy that Tom's Hardware could do for the community would be to push the OEM companies into more of a modular notebook design. If I am going to buy into the idea that I can replace my desktop with my laptop, then I want to know that I can upgrade components in my laptop like I can with my desktop.
My two cents...
Mike