James MacAonghus, an analyst at Aqute Research, points out that AdSense is becoming an increasingly smaller part of Google's income.

He looks at what's left of AdSense revenue, after subtracting the total acquisition costs. The acquisition costs include marketing and administration, but mostly comprise the share of AdWords revenue that Google pays to content providers.

As a proportion of Google's revenue, the retained AdSense revenue has dropped over recent years:

2002: 3%
2003: 11%
2004: 17%
2005: 14%
2006: 12%
2007: 7%
2008 (so far): 5%

On the other hand, revenue from Google owned websites (search, YouTube, Google Groups, Gmail, etc) is currently 91% of total revenue.

Acqute Research Blog: AdSense less important...
http://blog.aqute.com/aquteresearch/2008/08/adsense-less-im.html