This move by the regime has two objectives. The first is to limit the ability of activists to organize. Tuesday's protests were largely organized over Facebook and other social media sites. By denying the activists a communication tool, the regime hopes to limit their organizational ability. Given the widespread publicity about the protests scheduled for tomorrow, this is highly unlikely to be effective.
Secondly, the shutdown is intended to slow the information coming out of Egypt. Given that policy elites are getting much of their information from Twitter, the shutdown will slow the response of the international community, giving the regime time to re-establish control, and delaying the immediacy of the impact casualties will have.