I've been spending some time reading up on the SQL 2005 Papers.
If you haven't taken a look, there is lots of interesting things to read. Well worth a browse espically the operations section. http://www.microsoft.com/technet/prodtechnol/sql/2005/library/default.mspx)
Anyway it would be nice to see consistant advice from MS about sql service accounts....

In the  'Setting Up Windows Service Accounts'  document is states

Microsoft recommends that you do not use the Network Service account for the SQL Server or SQL Server Agent services. Local User or Domain User accounts are more appropriate for these SQL services.
(http://msdn2.microsoft.com/en-us/library/ms143504.aspx)

Whereas in the 'Physical Database Storage Design' paper  it recommends 'Use the Windows Network Service Account for security.'
(http://www.microsoft.com/technet/prodtechnol/sql/2005/physdbstor.mspx)

Happy Reading
Steven