For some reason I can never remember how to enable logging for the Exchange Writer on Exchange 2007. Here is the Powershell command that does it for you:
For the Store Writer:
Set-EventLogLevel "MSExchangeIS\9002 System\Exchange Writer" -Level High
For the Replication Writer:
Set-EventLogLevel "MSExchange Repl\Exchange VSS Writer" -Level High
On a related note, if you have enabled VSS tracing for the OS, you can get the Exchange Writer to output trace information by restarting Store or the Replication Service.
** Note - These should work for Exchange 2010 as well.