I dont specify my product key in the actual built in task sequence, instead I add 2 tasks below the Tattoo section of the State Restore Phase.
Add - command line
Name - Register Windows Product Key
Command Line - cscript %windir%\system32\slmgr.vbs /ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX (Where XXXX etc is your product key)
Add another below that:
Add - command line
Name - Activate Windows
Command Line - cscript %windir%\system32\slmgr.vbs /ato
So not only do I specify a product key, I also activate it as part of the state restore phase.
Hope this helps!
Aaron.