Since there are two issues here lets look at both:
1. Reliability and/or robustness
2. Functionality and performance
1. Reliability and/or robustness. These determine how long a piece of equipment will continue to function as originally specified by its designers and manufacturers. With laptops the main factors are:[list=1][*]mechanical stress caused by constant handling[*]Electrical stress due to power cycling[*]Thermal stress[/list=1]
All these stresses are going to affect the lifespan of the LCD, hard disk drives and to a lesser extent connectors and/or circuit boards. If you look at the MTBF (mean time before failure) for each component the LCD has 30-50k hrs, disks 300-500k hrs, and 100-200k hrs for PCB/componentry (as per MIL-HDBK-217). Obviously the LCD has the shortest statistical life of approx 3.5 years.
2. Functionality and performance. This is how long will the system run the software you intend to use allowing for upgrades and new functionality. This is limited by the amount to which you can upgrade and/or expand the hardware for a given system. I am currently using a Toshiba Satellite PIII - the CPU is not expandable, I have maxed out memory to 192Mb and my disk is currently 3/4 full at 20Gb. It could run WinXP but is it worth it - no because the real limiting factor is the applications I run need more memory. When I look at the 8890 I have considered it can go to 2Gb of memory (wish it was 4Gb) and the CPU might be able to be changed to a slightly better version than the 3.2GHz (larger cache maybe). I know Intel is close to the physical limits on the Northwood P4 chip so it is unlikely it will go much higher. However 3.2GHz is top end right now and compared to my PIII @ 500 Mhz is bliss (The PIII at one stage was top end too).
As design and TTM (time to market) cycles get shorter the time between newer/faster/larger products. It does not make the existing product any less effective - until it does not do what you need it to do. Most of the time you will never see the machinery break down, it is more the software will overtake the capacity of what you have right now. I still have Sun Sparc Station 2's running SunOS 4.1.3_U1. That is 1990's hardware still running perfectly. If I try to run the latest Solaris 8 on it - well it is a bit slow. It does not make my SS2 any less useful, it is just I can't do all the things like video editing and Trek gaming I would like to do now.
Most companies look at a 3 year payback period on their hardware (even the tax office allows you to depreciate your laptops in that time and the tax office is pretty tough on giving away anything). You can extend the life to 5 years if you have picked a system with enough expansion/headroom in it. You used to get 10 years from a product but with the growth in software features, 3 years is really the limit. Hey your LCD is only designed to last 3.5 years so at least you know when to sell your Sager before it breaks and buy the new 9000 series with the M50+ video card, 10 Gb of RAM, direct bio input and flexible LCD
Oh and btw Sagers should still rock in 3 years time