From the FAQ:
How fast will Rosetta impact the surface?
One of the key features of the trajectory design is to
minimise the spacecraft's relative velocity at impact. The
current scenario predicts that the impact velocity will be
around 90 cm/s, around walking pace. It is worth keeping in
mind that Rosetta was not designed as a lander, and some of
its appendages including the 32m-wide solar panels will be
damaged by the impact. This energy dissipation will very
likely ensure that the escape velocity will not be exceeded
during any bounce, thus preventing Rosetta from returning
to orbit after impact with Comet 67P/C-G.
(I read from this that it is highly unlikely that the comet trajectory is affected in a noticeable way by the impact. See also the answer from symmetricsaurus above. This would qualify as a good FAQ, IMHO.)