  Within the lowest-order Born approximation, we present an exact calculation
of the time dynamics of the spin-boson model in the Ohmic regime. We observe
non-Markovian effects at zero temperature that scale with the system-bath
coupling strength and cause qualitative changes in the evolution of coherence
at intermediate times of order of the oscillation period. These changes could
significantly affect the performance of these systems as qubits. In the biased
case, we find a prompt loss of coherence at these intermediate times, whose
decay rate is set by $\sqrt{\alpha}$, where $\alpha$ is the coupling strength
to the environment. These calculations indicate precision experimental tests
that could confirm or refute the validity of the spin-boson model in a variety
of systems.
