  The Rossi X-ray Timing Explorer has observed five outbursts from the
transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We
present a pulse timing study of the most recent outburst and compare it with
the previous timing solutions. The spin frequency of the source continues to
decrease at a rate of (-5.5+/-1.2)x10^-18 Hz/s, which is consistent with the
previously determined spin derivative. The spin-down occurs mostly during
quiescence, and it is most likely due to the magnetic dipole torque from a B =
1.5x10^8 G dipolar field at the neutron star surface. We also find that the 2
hr binary orbital period is increasing at a rate of (3.80+/-0.06)x10^-12 s/s,
also consistent with previous measurements. It remains uncertain whether this
orbital change reflects secular evolution or short-term variability.
