According to an article on Space Today, Russia has announced that they have agreed on plan with NASA to keep the ISS manned while the Shuttle is grounded.

The basics are that the existing crew will return to earth on the currently attached Soyuz craft in April. At about the same time, a new Soyuz will arrive at the ISS carrying two new crew, one Russian cosmonaut and one NASA astronaut.

If the shuttle remains grounded, then another Soyouz exchange will take place later in the year to bring the next crew up.