Laptop vs Desktop
Before I make my comments, I would like to declare my underlying assumptions:
1) a 970 dual AlBook would also have improved FSB speed and could address 2 GB RAM minimum
2) in real-world testing, the 970 AlBook dualie would outperform the current 1.42 DP tower in every conceivable speed test and render video like a champ
3) Panther as 64bit native and taylor made for the 970's (future app compatability)
All that assumed, I would suggest:
Is $3700 an insane price for a laptop? Perhaps not...
A dual processor 970 17" AlBook would be a true "desktop replacement." With a FW800 RAID array of external drives (easily set up in OS X) a video editor could use the unit as his post studio machine and be productive--then unplug and go to make his final presentation (If he's flying home from a shoot, he captures his video in an offline format and has a scratch version edited and ready before the plane even lands).
If you need actual PCI card space for some tasks, just get a Magma chassis--You can have an entire TOWER of cards hooked up to your AlBook (Mac RT, ProTools, MOTU stuff, etc) through the CardBus slot.
You could spend $4k on a well config'd tower + 20" display AND buy a cheaper laptop for $2K more (for presentations, etc),
or
Buy the laptop for $4k and use the remaining $2k for your Magma chassis and related peripherals.
The benefit of two separate systems would be the ability to work on two separate projects at once, I suppose. But if you are ONE PERSON working on a system, that may not afford you much of an advantage. Having all of your applications in exactly the perfect configuration with you at all times is a blessing.
I have used my meek little Pismo 400 as my one and only system for two years now. Being able to unplug it from my peripherals and jam it into a bag and go--man, that is a dream. No second system to keep sync'ed with the first... all my emails, applications, and documents travelling with me... the WORLD being my "home office" environment. Those kind of simple conveniences add up to make portable computing EXREMELY appealing, no matter the price or performance hit.
For home users who want to surf the net and play Quake, it doesn't make any sense, but to the independent or corporate businessman it may be a perfectly logical choice. How much PRODUCTIVE WORK (i.e., what your clients PAY you to do) you can get accomplished is more important than raw ghz to $ ratios. Paying an extra $1-2k for complete portability might seem foolhardy--but if the result nets you even ONE additional client per YEAR, you have more than made up the difference.