  SN 2001em, identified as a Type Ic supernova, has recently been detected in
the radio and X-rays, $\gtrsim$ 2 yr after the explosion. The high luminosities
at such late times might arise from a relativistic jet viewed substantially
off-axis that becomes visible only when it turns mildly relativistic and its
emission is no longer strongly beamed away from us. Alternatively, the emission
might originate from the interaction of the SN shell with the circumstellar
medium. We find that the latter scenario is hard to reconcile with the observed
rapid rise in the radio flux and optically thin spectrum,
$F_\nu\propto\nu^{-0.36\pm 0.16}t^{1.9\pm 0.4}$, while these features arise
naturally from a misaligned relativistic jet. The high X-ray luminosity
provides an independent and more robust constraint -- it requires $\sim
10^{51}$ erg in mildly relativistic ejecta. The source should therefore
currently have a large angular size ($\sim$ 2 mas) which could be resolved in
the radio with VLBA. It is also expected to be bipolar and is thus likely to
exhibit a large degree of linear polarization ($\sim 10%-20%$). The presence of
a relativistic outflow in SN 2001em would have interesting implications. It
would suggest that several percent of SNe Ib/c produce mildly relativistic
jets, with an initial Lorentz factor $\Gamma_0\gtrsim 2$, while the fraction
that produce GRB jets (with $\Gamma_0\gtrsim 100$) is $\sim 100$ times smaller.
This could considerably increase the expected number of transients similar to
orphan GRB afterglows in the radio, and to a lesser extent in the optical and
X-rays, if there is a continuous distribution in $\Gamma_0$. Furthermore, this
may give further credence to the idea that core collapse SNe, and in particular
SNe Type Ib/c, are triggered by bipolar jets.
