There is a known issue with DS6.9 SP6 and WinPE4 which means that the PXE environment variables used in this article won't be set. To resolve this please take a look at http://www.symantec.com/docs/TECH213143 and replace your startutl.bat batch file with the revised version in the tech note.
This is important for sites who use their DS6.9 PXE servers as image stores, and who therefore map a drive to \\%ALTIRIS_PXE_IP%\images in Bootwiz to assist this process transparently across multiple PXE servers.
My google-fu failed to pick this tech note up and it took a fast response Altiris support ticket to track this one down. I've updated my "WAN Nirvana" article to reflect this.