Try the following format:
%Y-%m-%dT%H:%M:%S.%fZ
For example:
>>> datetime.datetime.strptime('2012-05-29T19:30:03.283Z', '%Y-%m-%dT%H:%M:%S.%fZ')
datetime.datetime(2012, 5, 29, 19, 30, 3, 283000)
The Z in the date just means that it should be interpreted as a UTC time, so ignoring it won’t cause any loss of information. You can find this information here: http://www.w3.org/TR/NOTE-datetime