We determined the flux ratios of the heavy and eccentric planet XO-3b to its parent star in the four Infrared Array Camera bands of the Spitzer Space Telescope: 0.101% +- 0.004% at 3.6 {mu}m; 0.143% +- 0.006% at 4.5 {mu}m; 0.134% +- 0.049% at 5.8 {mu}m; and 0.150% +- 0.036% at 8.0 {mu}m. The flux ratios are within -2.2, 0.3, -0.8, and -1.7sigma of the model of XO-3b with a thermally inverted stratosphere in the 3.6 {mu}m, 4.5 {mu}m, 5.8 {mu}m, and 8.0 {mu}m channels, respectively. XO-3b has a high illumination from its parent star (F{sub p} {approx} (1.9-4.2) x 10{sup 9} erg cm{sup -2} s{sup -1}) and is thus expected to have a thermal inversion, which we indeed observe. When combined with existing data for other planets, the correlation between the presence of an atmospheric temperature inversion and the substellar flux is insufficient to explain why some high insolation planets like TrES-3 do not have stratospheric inversions and some low insolation planets like XO-1b do have inversions. Secondary factors such as sulfur chemistry, atmospheric metallicity, amounts of macroscopic mixing in the stratosphere, or even dynamical weather effects likely play a role. Using the secondary eclipse timing centroids, we determined the orbital eccentricity of XO-3b as e = 0.277 +- 0.009. The model radius-age trajectories for XO-3b imply that at least some amount of tidal heating is required to inflate the radius of XO-3b, and the tidal heating parameter of the planet is constrained to Q{sub p} {approx}< 10{sup 6}.