Google becomes world's first $100 billion brand
techcombo.com
techcombo.com
I found page 6 of this pdf: http://www.millwardbrown.com/Sites/Optimor/Media/Pdfs/en/Bra...
The summary is that it's the NPV of the brand's future earnings, multiplied by the fraction of that revenue due to the "brand" itself and not location/price/etc (this is estimated by market research experts), multiplied by an estimate of future growth (more market research hand-waving).
http://royal.pingdom.com/2009/04/30/the-money-made-by-big-te...
With a company like Google the rules are different because the value is in the domain name. I think they're equating brand name with domain name, which makes for an apples to oranges comparison with companies like HP.