Most of the friction which is slowing the Earth's rotation is...

Most of the friction which is slowing the Earth's rotation is due to the tidal pull of water in the ocean by the moon. In about a billion years though, the expansion of the sun will have caused the evaporation of all of Earth's oceans, so it's unlikely that the Earth will ever be completely spun down. In any case, the planet will be a waterless, inhospitable inferno.

Interesting stuff though.