It depends on the exact moment of the power loss, as if it is during the downloading period, then no, the Wii console will not get "bricked" and only have to restart the update process again to complete it.:)
However, if the power failure was to occur during the time in which the update information is being written to the system files, then yes, it is possible that the data could be damaged or corruputed and that could brick the console.
But since the rewritting of the system files is more or less instant, the odds of having it occur are relatively remote, so that when a console is bricked, it is usually due to others factors, such as ill advised firmware modifications, or a hardware problem such as the Flash memory of the console failing and is no longer able to read the system files correctly.
