  We report the results of infrared (8 micron) transit and secondary eclipse
photometry of the hot Neptune exoplanet, GJ436b using Spitzer. The nearly
photon-limited precision of these data allow us to measure an improved radius
for the planet, and to detect the secondary eclipse. The transit (centered at
HJD = 2454280.78149 +/-0.00016) shows the flat-bottomed shape typical of
infrared transits, and it precisely defines the planet-to-star radius ratio
(0.0839 +/-0.0005), independent of the stellar properties. However, we obtain
the planetary radius, as well as the stellar mass and radius, by fitting to the
transit curve simultaneously with an empirical mass-radius relation for
M-dwarfs (M=R). We find Rs=Ms=0.47 +/-0.02 in solar units, and Rp=27,600
+/-1170 km (4.33 +/-0.18 Earth radii). This radius significantly exceeds the
radius of a naked ocean planet, and requires a gasesous hydrogen-helium
envelope. The secondary eclipse occurs at phase 0.587 +/-0.005, proving a
significant orbital eccentricity (e=0.15 +/-0.012). The amplitude of the
eclipse (5.7 +/-0.8e-4) indicates a brightness temperature for the planet of
T=712 +/-36K. If this is indicative of the planet's physical temperature, it
suggests the occurrence of tidal heating in the planet. An uncharacterized
second planet likely provides ongoing gravitational perturbations, to maintain
GJ436b's orbit eccentricity over long time scales.
