  We report the discovery of SDSSJ1049+5103, an overdensity of resolved blue
stars at (\alpha_{2000}, \delta_{2000}) = (162.343, 51.051). This object
appears to be an old, metal-poor stellar system at a distance of 45 +/- 10 kpc,
with a half-light radius of 23$\pm 10$ pc and an absolute magnitude of M_V =
-3.0^{+2.0}_{-0.7}. One star that is likely associated with this companion has
an SDSS spectrum confirming it as a blue horizontal branch star at 48 kpc. The
color-magnitude diagram of SDSSJ1049+5103 contains few, if any, horizontal or
red giant branch stars, similar to the anomalously faint globular cluster AM 4.
The size and luminosity of SDSSJ1049+5103 places it at the intersection of the
size-luminosity relationships followed by known globular clusters and by Milky
Way dwarf spheroidals. If SDSSJ1049+5103 is a globular cluster, then its
properties are consistent with the established trend that the largest radius
Galactic globular clusters are all in the outer halo. However, the five known
globular clusters with similarly faint absolute magnitudes all have half-mass
radii that are smaller than SDSSJ1049+5103 by a factor of $\gtrsim$ 5. If it is
a dwarf spheroidal, then it is the faintest yet known by two orders of
magnitude, and is the first example of the ultra-faint dwarfs predicted by some
theories. The uncertain nature of this new system underscores the sometimes
ambiguous distinction between globular clusters and dwarf spheroidals. A simple
friends-of-friends search for similar blue, small scalesize star clusters
detected all known globulars and dwarfs closer than 50 kpc in the SDSS area,
but yielded no other candidates as robust as SDSSJ1049+5103.
