Nearly any Intel Centrino notebook with a Pentium M cpu will go at least 3.5- 4 hours using wi-fi all the time (or playing DVD's) or up to 5-6 hours in some cases when doing simple programs and not using wi-fi (like Microsoft Office).
As I told you, I have a 15.4" widescreen Compaq X1000 (X1360US) Pentium 1.4 mhz --that is a 'Banias' Pentium M and had 1mb L2 system cache --the new Pentium M's with a code of 715 or higher have 2mb L2 system cache.
You will find better values overall with 15" square and 15.4" widescreen notebooks as that is what is selling at the consumer level -- except the ultralight models from Fujitsu (13.3") and Sony go to smaller LCD screens.
Again, keep in mind that consumer models will normally have 1 year warranty while the HP business, Toshiba business and IBM (all business) and the Dell business models should all have 3 year warranties -- so you have to weigh that in the pricing.
If you find an IBM you want buy it as it is more durable and has a better keyboard than nearly any other notebook. They stick to 14.1" and smaller and charge more for what you get but you don't seem to want a larger LCD than 14.1"
You can read about the sister models I have at X1000forums.com (10,000 users) as well as all the discounts you can get --
You could get something like this for under $1500 at 6.5 pounds (and it will fit in a 15" Targus notebook backpack).
Pentium 725 1.6 Pentium M cpu (2mb L2 system cache)
256mb RAM (get more on your own)
64mb video -- ATI 9200
60mb 5400rpm hard drive
CD-RW/DVD ROM optical drive
Intel 2200 b/g wifi
15.4" XGA LCD
3 USB 2.0 ports, 1 firewire port
SD card slot (for newer digital cameras)
Also, I really like the Toshiba M35 series --it is nearly identical to the above but weighs 6.2 pounds with the 15.4" LCD ....
Dell has the 8600 line similar to the above with a 15.4" LCD at 6.9 pounds.....
All of these consumer models have a 1 year limited warranty standard and Windows XP home.
One important thing is the keyboard --the IBM's and the Compaq X1000/HPZT3000 have some of the best.
If you want to see all three of three of these 15.4" LCD models to see whether they are too big for you or not go to Circuit City to see the prebuilt HP ZT3380, the Compaq 1480US, and the Toshiba M35S456 ...
These cost more retail as they all have 512mb RAM, a DVD burner (which you probably don't need and they cost 200 more just for this) and they have an Intel Pentium M 1.7 735 which is $125 more than the 1.6 Pentium M you can order custom
You can also view all 3 of these models at CircuitCity.com if you wish -- choose the filter with Pentium M (Centrino) cpu and you will see only notebooks they sell with a Pentium M cpu (and you can do this on Cnet also)