PSR J1738+0333 is a 5.85-ms pulsar in a binary system with an orbital period of 8.5 hours and a companion white dwarf with a mass of about 0.2 solar masses. This pulsar has been timed with Arecibo's 305-m telescope for the last 3 years; in this talk I present the results of this project. This is one of the most precisely timed pulsars ever, with rms timing residuals of the order of 200 ns per WAPP. This timing precision has yielded impressive results: a) the orbit is very circular, it does not depart from a circle by more than 80 microns; b) we can measure the proper motion and parallax with remarkable precision; c) we can already measure the orbital decay of this binary system due to the emission of gravitational waves, albeit with low precision; it is -(5 ± 3)*10-14 s/s, indicating that the orbital period becomes 2 micro-seconds shorter every year. The detection of the companion white dwarf star at optical wavelengths has allowed an estimate of its mass (0.2 solar masses) and of the mass ratio of the system, 8.1 ± 0.3. The pulsar mass is therefore about 1.6 solar masses. The expected rate of orbital decay due to the emission of gravitational waves can therefore be calculated, it is -6 * 10-14 s/s, in agreement with the observed value. The absolute difference between predicted and observed values is so small that this system already introduces the tightest constraints ever on dipolar gravitational wave emission (predicted by all alternative theories of gravitation, general relativity predicts only quadrupolar emission). I finally discuss the prospects of this project. Continued timing of PSR J1738+0333 over the course of 5 to ten years will measure the orbital decay tens of times more precisely than at the present time. This might rule out the last class of viable alternative theories to general relativity (tensor bi-scalar gravitation), or lead to the discovery of dipolar gravitational emission, which would take physics beyond Einstein. In either case, PSR J1738+0333 will definitely be, by far, the best known system for constraining alternative theories of gravitation.