We present Spitzer infrared (IR) photometry and spectroscopy of the lensed Lyman break galaxy (LBG) MS 1512-cB58 at z = 2.73. The large (factor similar to 30) magnification allows for the most detailed IR study of an L(UV)*(z = 3) LBG to date. Broadband photometry with IRAC (3-10 mu m), IRS (16 mu m), and MIPS (24, 70, and 160 mu m) was obtained, as well as IRS spectroscopy spanning 5.5-35 mu m. A fit of stellar population models to the optical/near-IR/IRAC photometry gives a young age (similar to 9 Myr), forming stars at similar to 98 M(circle dot) yr(-1), with a total stellar mass of similar to 10(9)M(circle dot) formed thus far. The existence of an old stellar population with twice the stellar mass cannot be ruled out. IR spectral energy distribution fits to the 24 and 70 mu m photometry, as well as previously obtained submillimeter/ millimeter data give an intrinsic IR luminosity L(IR) (1-2); 10(11) L(circle dot) and a star formation rate (SFR) similar to 20Y40M(circle dot) yr(-1). The ultraviolet (UV) derived SFR is similar to 3-5 times higher than the SFR determined using LIR or L(H alpha) because the red UV spectral slope is significantly overpredicting the level of dust extinction. This may suggest that the assumed Calzetti starburst obscuration law is not valid in young LBGs. We detect strong line emission from polycyclic aromatic hyrdrocarbons (PAHs) at 6.2, 7.7, and 8.6 mu m. The line ratios are consistent with ratios observed in both local and high- redshift starbursts. Both the PAH and rest- frame 8 mu m luminosities predict the total LIR based on previously measured relations in starbursts. Finally, we do not detect the 3.3 mu m PAH feature. This is marginally inconsistent with some PAH emission models, but still consistent with PAH ratios measured in many local starforming galaxies.
