  We report the first detection of an optical afterglow of a GRB (060108) that
would have been classified as 'dark' in the absence of deep, rapid ground-based
optical imaging with the 2-m robotic Faulkes Telesscope (FTN). Our
multiwavelength analysis reveals an X-ray light curve typical of many Swift
long GRBs (3-segments plus flare). Its optical afterglow, however, was already
fainter than the detection limit of the UVOT within 100s of the burst. Optical
imaging in BVRi' filters with the FTN began 2.75 minutes after the burst and
resulted in the detection of the optical afterglow at 5.3 minutes, with a UKIRT
K-band identification at ~45 mins. R and i'-band light curves are consistent
with a single power law decay in flux, F(t) prop t^-a where a=0.43+/-0.08, or a
2-segment light curve with a steep decay a_1 <0.88, flattening to a_2 ~ 0.31,
with evidence for rebrightening at i' band. Deep VLT R-band imaging at ~12 days
reveals a faint, extended object (R ~23.5 mag) at the location of the
afterglow. Although the brightness is compatible with the extrapolation of the
a_2 slow decay, significant flux is likely due to a host galaxy. This implies
that the optical light curve had a break before 12 days, akin to what observed
in the X-rays. We derive a maximum photometric redshift z<3.2 for GRB 060108
and a best-fitting optical-to-X-ray SED at 1000 s after the burst consistent
with a power law with index beta_OX = 0.54 and a small amount of extinction.
The unambiguous detection at B-band and the derived photometric redshift rule
out a high redshift as the reason for the optical faintness of GRB 060108.
Instead, the hard opt/X-ray spectral index confirms it as one of the
optically-darkest bursts detected and with modest host extinction explains the
UVOT non-detection (abridged).
