Leo meant to say an Intel 'Centrino' gives better battery life, thickness and weight than a Pentium 4M notebook which are ideal only for heavier 17" LCD desktop replacements (Centrino Pentium M cpu not Celeron).
Don't get a Celeron Pentium M or Celeron Pentium 4M --they are decontented chips and you are putting yourself and the end of the productivity curve right at the point of purchase,
The processor is the main determinant of battery life (assuming the brand are using the same size of battery (and lithium)- typically 8 cell in Pentium M's and 12 cell in Pentium 4M's.
You will get 3.5 - 5 hours of battery life on a HPZT3000/Compaq X1000 15.4" widescreen (see X1000forums.com and hpshopping.com) or a Toshiba M35 series with the same cpu (Pentium M), same size 15.4" LCD and similar weight (6.5 pounds versus 6.2 pounds).
Get an Athlon 64 if you will use Linux now or want to to use the notebook heavily beyond when 64bit Longhorn windows apppears likely in 2007 (3 years out). The Athlon 64 is a better choice than the Pentium 4M but still thicker and heavier and less battery life than the Pentium M Centrino.
Make sure you determine whether you need dedicated video memory or not before buying any notebook as you can't go back later.
I nearly bought a Toshiba M35 but got the Compaq X1000/HPZT3000 -- go with either Toshiba/HP -- order HP custom at hpshopping for best specs and discounts ($100 mail in rebate through 12/1/04 -- get pdf file on mail in rebates link on hpshopping homepage).
Sony is overpriced and their service is rated dead last-- if you want one of their ultralights or expensive media systems they may be the only alternative but in the $1,000-$2,000 notebook sweet spot I would not get Sony.