Exchange Server 2007 Service Pack 3 introduces a new feature that monitors the Information Store service for long running transactions. Transactions are now timed and if a transaction lasts longer than 60 seconds (hardcoded), it’s considered to be timed out. The transaction isn’t terminated; it’s just flagged as taking too long.
This monitoring has been added to help report on the health of the Information Store. There is a myriad of reasons for long running transactions. Some of these reasons are explained in Understanding the Performance Impact of High Item Counts and Restricted Views.
Update: Timeout detection and reporting was introduced in Exchange 2010 RTM and backported to Exchange 2007 SP3. See Understanding the Exchange 2010 Store -> Time-Out Detection and Reporting.