This is after updating to 10.6.8
Ok for the app store sign in issues here is a simple fix.
First go to Extra folder and check your com.apple.Boot.plist (
or Library/Preferences/System Configuration if its not in extra folder)
to make sure that this string is in it,
<key>EthernetBuiltIn</key>
<string>Yes</string>
If it isn't add it restart and see if that fixes it.
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If it is in there or if adding it didn't help you need to remove some other items
Go to,
System preferences/Network
You'll want to highlight each one and remove it by clicking the - tab at bottom of box.
Then go to,
Library/Preferences/System Configuration
and delete or place on desktop the NetworkInterfaces.plist
Then restart system.
After you restart you will want to go back to System Preferences/Network and
add all the things you removed starting with ethernet.
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If those steps don't work then try;
adding the following to your com.apple.Boot.plist:
<key>PCIRootUID</key>
<string>1</string>
Then remove the NetworkInterfaces.plist and rebooted
What I had noticed is that the ethernet was labeled en0 and Airport card was en1 and after update the numbers got switched. It was only switched in the NetworkInterfaces.plist but stayed the same in every other settings so it was give conflicting things while trying to log into the app store.
Thanks Lnx2mac