And just a quick note for all of you who are struggling too, I initially after the update did all that deleting the Network Interfaces in Network Prefs/deleted the NetworkPreferences.plist stuff & rebooting & then re-added the Network Interfaces stuff but none of it worked ! So then I tried what Paulo said of just adding EthernetBuiltIn to my com.apple.Boot.plist in /Extra/ & it worked straightaway !!
BUT as an afterthought, I then decided to restore a backup that I had made just BEFORE updating to 10.6.8, and again ran the update & when that was all finished I just made the changes to com.apple.Boot.plist WITHOUT having to do ANY of that deleting Network Interfaces/NetworkPreferences.plist stuff, and it worked straightaway !!
So for me & hopefully others, I proved that you just have to make the changes to the Boot.plist and the AppStore should work again !!
Damn i updated to 10.6.8 and im having the same no GUID available error when trying to log into the App Store... i really thought this option was going to help me (i did tried the other options like deleting de net adapter and deleting the net cache file and rebooting) but after adding the EthernetBuiltIn key in my com.apple.Boot.plist (in my case located at /Library/Preferences/SystemConfiguration/com.apple.Boot.plist) it still gives me that no GUID available error... :'( anyone else has bumped with this one???