When I updated my Windows 8 to the technical preview it was not automatically activated as described in the Microsoft documentation.
The following process will take care of the activation issue on in-place updated Windows 10 installations:
1. Open Administrative command prompt
2. Type: SLUI 3
3. Enter the product key Microsoft provided for recovery scenarios: PBHCJ-Q2NYD-2PX34-T2TD6-233PK
This will get rid of the Activation Message (at least until April of 2015)