  Galaxies' rest-frame ultraviolet (UV) properties are often used to directly
infer the degree to which dust obscuration affects the measurement of star
formation rates. While much recent work has focused on calibrating dust
attenuation in galaxies selected at rest-frame ultraviolet wavelengths, locally
and at high-$z$, here we investigate attenuation in dusty, star-forming
galaxies (DSFGs) selected at far-infrared wavelengths. By combining
multiwavelength coverage across 0.15--500\,$\mu$m in the COSMOS field, in
particular making use of {\it Herschel} imaging, and a rich dataset on local
galaxies, we find a empirical variation in the relationship between rest-frame
UV slope ($\beta$) and ratio of infrared-to-ultraviolet emission ($L_{\rm
IR}/L_{\rm UV}\equiv\,IRX$) as a function of infrared luminosity, or total star
formation rate, SFR. Both locally and at high-$z$, galaxies above
SFR$\gt$50\,M$_\odot$\,yr$^{-1}$ deviate from the nominal $IRX-\beta$ relation
towards bluer colors by a factor proportional to their increasing IR
luminosity. We also estimate contamination rates of DSFGs on high-$z$ dropout
searches of $\ll1$\%\ at $z\lt4-10$, providing independent verification that
contamination from very dusty foreground galaxies is low in LBG searches.
Overall, our results are consistent with the physical interpretation that
DSFGs, e.g. galaxies with $>50$\,M$_\odot$\,yr$^{-1}$, are dominated at all
epochs by short-lived, extreme burst events, producing many young O and B stars
that are primarily, yet not entirely, enshrouded in thick dust cocoons. The
blue rest-frame UV slopes of DSFGs are inconsistent with the suggestion that
most DSFGs at $z\sim2$ exhibit steady-state star formation in secular disks.
