Read the publication here http://sqlcat.com/technicalnotes/archive/2008/12/09/diagnosing-transaction-log-performance-issues-and-limits-of-the-log-manager.aspx

Also note the new SQL Server 2008 transaction log specification:

1. Amount of “in-flight” I/O Limit.

a. SQL Server 2008: limit of 3840K at any given time

b. Prior to SQL Server 2008: limit of 480K at any given time