That sounds quite plausible.
..and quite slow.
Like the person who mentioned the iphone processor - the data read/write rates of the ipod hard drives are designed around the most data-intensive task that the ipod would ever create. This would be a VERY low resolution video file (a BIG HD size video file is a few mbps TOPS.)
New desktop SATA drives are "rated" at 3gbps in theory, and even SLOW EIDE drives get 60mbps in real world data rate tests.
Most data it used in bursts these days, and Apple would be able to solve this issue by writing bursts of data to say.. 4-8bg of flash data, then slowly transfer this over to the "ipod" hard drive.
This wouldn't help you with LOADING data however, which would be extremely slow. They could develop a large-scale cache system using the same 4-8bg of flash memory for loading files, but this would still require "learning" your normal programs, and wouldn't be an exact science.
Keep in mind that they have HUGE supplies and price cuts for flash and micro HDD memory through their ipod markets, so if there was a way that they could take advantage of this, I'm sure they'd like to.
As far as a new design goes - you could just take the iphone's approach to the laptop world. Instead of the old school clamshell style computer, you could just have a giant, beautiful, touch screen laptop. Don't confuse this with a tablet - combined with a nice "touch" version of OS X leopard, you could have a keyboard pop up on the bottom of the screen when you need it. Hell, you could even use a small overlay area for the touch pad and buttons! They've already patented the touch technology used in the iphone/ipod touch, why not take it a step further? Cover it in the same optical-quality glass material, use a strong aluminum frame, and you've got one BADASS computer.
You could even have a fold-out aluminum cover that latches over the screen like a normal laptop (however, it simply opens up 180 degrees and folds flush with the BOTTOM of the case when it's in use.)
WHYYYYYY doesn't this computer exist? LED backlight, high resolution.. ugh, do it apple!
Think of a FULL SIZE iphone screen - how beautiful would that be? Besides, you could play around with it like the guys in Minority Report - grab ahold of applications, toss them away, flick to resize windows, etc etc etc.
Ok, I've gotta stop thinking about this before I start writing a letter to their R&D department.