As expressed, this runs the risks of inaccuracies for very long wavelengths and warnings about overflow error for very small ones. 
(e.g., I get an overflow warning for `planck(1.*u.AA, 1e4*u.K)`, and results are less accurate for, e.g., `planck(10.*u.m, 1.e10 * u.K)`. You might want to catch the overflow (since it is not actually physically meaningful); the inaccuracies would only seem to be an issue if people use `float32` (note that this *is* possible with quantities).