Here are some comments.
http://codereview.appspot.com/950/diff/1/3
File appengine_django/serializer/python.py (right):
http://codereview.appspot.com/950/diff/1/3#newcode80
Line 80: TIME_FORMAT = "%H:%M:%S"
I think it makes more sense to define a single constant DATETIME_FORMAT (or at
least define that here too, as the concatenation of the other two).
http://codereview.appspot.com/950/diff/1/3#newcode130
Line 130: if isinstance(field, db.DateTimeProperty):
I'm reluctant to see a growing list of
 if isinstance(field, X): ...
blocks here. I think it makes more sense to monkey-patch the Property
subclasses (with the eventual expectation that these changes will be adopted by
the appengine core).
Especially since DateProperty and TimeProperty will surely be next...
http://codereview.appspot.com/950/diff/1/3#newcode134
Line 134: field_value = d
Any reason you aren't writing
 field_value = datetime.datetime(*(t)[0:6])
?
http://codereview.appspot.com/950/diff/1/3#newcode135
Line 135: except (ValueError, TypeError), e:
I'm uncomfortable casting such a wide net. I'd much rather only try errors in
the strptime() call, and guard the block with a typecheck on the value (checking
that it is a unicode instance).
http://codereview.appspot.com/950/diff/1/2
File appengine_django/serializer/xml.py (right):
http://codereview.appspot.com/950/diff/1/2#newcode142
Line 142: if isinstance(field, db.DateTimeProperty):
Why the duplicated code? I guess this is another argument for monkey-patching
the class... :-)