  We present Spitzer/IRS mid-infrared spectra for 15 gravitationally lensed, 24
micron--selected galaxies, and combine the results with 4 additional very faint
galaxies with IRS spectra in the literature. The median intrinsic 24 micron
flux density of the sample is 130 microJy, enabling a systematic survey of the
spectral properties of the very faint 24 micron sources that dominate the
number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra
(32%) show the strong mid-IR continuua expected of AGN; X-ray detections
confirm the presence of AGN in three of these cases, and reveal AGNs in two
other galaxies. These results suggest that nuclear accretion may contribute
more flux to faint 24 micron--selected samples than previously assumed. Almost
all the spectra show some aromatic (PAH) emission features; the measured
aromatic flux ratios do not show evolution from z=0. In particular, the high
S/N mid-IR spectrum of SMM J163554.2+661225 agrees remarkably well with
low--redshift, lower--luminosity templates. We compare the rest-frame 8 micron
and total infrared luminosities of star--forming galaxies, and find that the
behavior of this ratio with total IR luminosity has evolved modestly from z=2
to z=0. Since the high aromatic--to--continuum flux ratios in these galaxies
rule out a dominant contribution by AGN, this finding implies systematic
evolution in the structure and/or metallicity of infrared sources with
redshift. It also has implications for the estimates of star forming rates
inferred from 24 micron measurements, in the sense that at z ~2, a given
observed frame 24 micron luminosity corresponds to a lower bolometric
luminosity than would be inferred from low-redshift templates of similar
luminosity at the corresponding rest wavelength.
