  We present the first results from a long (51 ks) XMM-Newton observation of
the Galactic X-ray binary Swift J1910.2-0546 in a intermediate state, obtained
during its 2012 outburst. A clear, asymmetric iron emission line is observed
and physically motivated models are used to fully describe the emission-line
profile. Unlike other sources in their intermediate spectral states, the inner
accretion disk in Swift J1910.2-0546 appears to be truncated, with an inner
radius of \rin$=9.4^{+1.7}_{-1.3}$\rg\ at a 90% confidence limit.
Quasi-periodic oscillations are also found at approximately 4.5 and 6 hz, which
correlates well with the break frequency of the underlying broad-band noise.
Assuming that the line emission traces the ISCO, as would generally be expected
for an intermediate state, the current observation of Swift J1910.2-0546 may
offer the best evidence for a possible retrograde stellar mass black hole with
a spin parameter $a< -0.32 cJ/GM^2$ (90% confidence). Although this is an
intriguing possibility, there are also a number of alternative scenarios which
do not require a retrograde spin. For example, the inner accretion disk may be
truncated at an unusually high luminosity in this case, potentially suffering
frequent evaporation/condensation, or it could instead be persistently
evacuated through mass loss in a relativistic jet. Further observations are
required to distinguish between these different interpretations.
