The json serialization is broken on F17.  In F17, simplejson jumped to version 2.5.2 (from 2.16 in F16).  In 2.5.2, support for collections.namedtuples is added which intruduces the problem.  The encode() detects namedtuples by doing hasattr(o, '_asdict') and them blindly call it without doing a callable() check.  The gofer Envelope object overrides __getattr__() so hasattr() is True but then encode() tries to call None.

This trace can be seen in agent.log:

2012-06-06 15:06:05,537 [ERROR][Actions] __call__() @ action.py:129 - 'NoneType' object is not callable
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/gofer/agent/action.py", line 127, in __call__
    self.target()
  File "/usr/lib64/gofer/plugins/pulpplugin.py", line 94, in heartbeat
    return self.send()
  File "/usr/lib64/gofer/plugins/pulpplugin.py", line 105, in send
    p.send(topic, ttl=delay, heartbeat=body)
  File "/usr/lib/python2.7/site-packages/gofer/messaging/producer.py", line 49, in send
    json = envelope.dump()
  File "/usr/lib/python2.7/site-packages/gofer/messaging/__init__.py", line 80, in dump
    return json.dumps(d, indent=2)
  File "/usr/lib64/python2.7/site-packages/simplejson/__init__.py", line 296, in dumps
    **kw).encode(obj)
  File "/usr/lib64/python2.7/site-packages/simplejson/encoder.py", line 228, in encode
    chunks = list(chunks)
  File "/usr/lib64/python2.7/site-packages/simplejson/encoder.py", line 509, in _iterencode
    for chunk in _iterencode_dict(o._asdict(), _current_indent_level):
TypeError: 'NoneType' object is not callable
2012-06-06 15:06:15,552 [ERROR][Actions] __call__() @ action.py:129 - 'NoneType' object is not callable
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/gofer/agent/action.py", line 127, in __call__
    self.target()
  File "/usr/lib64/gofer/plugins/pulpplugin.py", line 94, in heartbeat
    return self.send()
  File "/usr/lib64/gofer/plugins/pulpplugin.py", line 105, in send
    p.send(topic, ttl=delay, heartbeat=body)
  File "/usr/lib/python2.7/site-packages/gofer/messaging/producer.py", line 49, in send
    json = envelope.dump()
  File "/usr/lib/python2.7/site-packages/gofer/messaging/__init__.py", line 80, in dump
    return json.dumps(d, indent=2)
  File "/usr/lib64/python2.7/site-packages/simplejson/__init__.py", line 296, in dumps
    **kw).encode(obj)
  File "/usr/lib64/python2.7/site-packages/simplejson/encoder.py", line 228, in encode
    chunks = list(chunks)
  File "/usr/lib64/python2.7/site-packages/simplejson/encoder.py", line 509, in _iterencode
    for chunk in _iterencode_dict(o._asdict(), _current_indent_level):
TypeError: 'NoneType' object is not callable