Instead of "SyntaxError: future feature foobar is not 
defined", you will get "SyntaxError: from __future__ 
imports must occur at the beginning of the file", if 
you use a semicolon at the end of the line (which is 
valid for existing future-imports of course).

Python 2.4.3 (#2, Apr 27 2006, 14:43:58)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright", "credits" or "license" for 
more information.
>>> from __future__ import foobar;
  File "<stdin>", line 1
SyntaxError: from __future__ imports must occur at 
the beginning of the file
>>> from __future__ import generators;
>>>