First, datetime.utcfromtimestamp() drops fractional part of Decimal argument:

>>> import datetime
>>> from decimal import Decimal as D
>>> datetime.datetime.utcfromtimestamp(1425808327.307651)
datetime.datetime(2015, 3, 8, 9, 52, 7, 307651)
>>> datetime.datetime.utcfromtimestamp(D(1425808327.307651))
datetime.datetime(2015, 3, 8, 9, 52, 7)

Second, it works only with C implementation. With Python implementation Decimals are not supported at all.

>>> del sys.modules['datetime']
>>> sys.modules['_datetime'] = None
>>> import datetime
>>> datetime.datetime.utcfromtimestamp(D(1425808327.307651))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.4/datetime.py", line 1374, in utcfromtimestamp
    t, frac = divmod(t, 1.0)
TypeError: unsupported operand type(s) for divmod(): 'decimal.Decimal' and 'float'

In 2.7 Decimals are accepted and are not truncated.

>>> import datetime
>>> from decimal import Decimal as D
>>> datetime.datetime.utcfromtimestamp(D(1425808327.307651))
datetime.datetime(2015, 3, 8, 9, 52, 7, 307651)

So this can be considered as a regression.