2006 is showing the early signs of slow down for four reasons:
1. Once you have tons of revenue, its hard to keep high Year-over-Year (YoY) growth rates
2. The search marketplace is slowing down a bit due to saturation in the US and European markets (still plenty of growth in Asia though, but Yahoo is stronger there).
3. Google launched almost all the tricks in the bag during 2003, 2004 and 2005, the only remaining tricks are visual placement tricks and looser matching (i.e. more, less-relevant, ads on top of web results).
4. None of Google’s other products, other than web search that is, have decent “money” marketshare. Google’s Image Search is actually pretty large, but they have no ads there (will that change in Q3? possibly).