This article indicates Instant File Initialization is only for data files.

http://www.microsoft.com/technet/prodtechnol/sql/2005/physdbstor.mspx

Instant File Initialization

SQL Server 2005 added support for the Windows instant file initialization feature. By default, when a file is created or grows larger, the pages in that file are initialized by writing zeros before the file gets used. This overwrites any existing data that remains on the disk. Instant initialization is only used for data files (not log files) and is enabled when the account running SQL Server has the Windows SE MANAGE VOLUME NAME privilege, which is available only on Microsoft Windows XP, Windows Server 2003 or later versions. This occurs in five scenarios:

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During file creation

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CREATE DATABASE, including tempdb creation at server startup.

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RESTORE DATABASE

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During file modification

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ALTER DATABASE...MODIFY FILE.

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Modifications that result in autogrow activity.