The new Search in Exchange Server 2007
The MsExchangeTeam explain to us how the new search of Exchange Server 2007 works.
In Exchange Server 2007, we completely rewrote Search - we upgraded from MS-Search 2.0 to 3.0 (which SQL Server 2007 also uses) and we changed from a "crawl" model to an "always up to date" model - we also rewrote the indexing part to be more efficient in how it communicates with the Exchange Information Store - the end result is an 35x improvement in indexing speed.
Read more at source: http://msexchangeteam.com/archive/2006/10/11/429163.aspx
Ice Rocket : Exchange 2007, Search