Thanks.  I seem to still have the problem for some codec settings, while other ones give me:

gstavidemux.c(4443): gst_avi_demux_loop (): /GstPipeline:pipeline1/GstBin:bin3/GnlComposition:gnlcomposition2/GnlSource:gnlsource3/GstBin:bin5/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin0/GstAviDemux:avidemux14:
streaming stopped, reason not-negotiated

The terminal output is also exciting:

ERROR [ 9392] [0x   -487dd940] "<Pipeline at 0x9cff18c>"        pipeline          Jan 06 21:47:40      _errorObject: error from /GstPipeline:pipeline1/GstBin:bin3/GnlComposition:gnlcomposition2/GnlSource:gnlsource3/GstBin:bin5/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin0/GstAviDemux:avidemux10 (__main__.GstAviDemux): GStreamer encountered a general stream error. (gstavidemux.c(4443): gst_avi_demux_loop (): /GstPipeline:pipeline1/GstBin:bin3/GnlComposition:gnlcomposition2/GnlSource:gnlsource3/GstBin:bin5/pitivi+elements+singledecodebin+SingleDecodeBin:pitivi+elements+singledecodebin+singledecodebin0/GstAviDemux:avidemux10:
streaming stopped, reason not-negotiated) (/usr/lib/pitivi/python/pitivi/log/loggable.py:30)
Traceback (most recent call last):
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 160, in _cancelButtonClickedCb
    self._shutDown()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 82, in _shutDown
    self.removeRecordAction()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 193, in removeRecordAction
    self.renderaction.deactivate()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 146, in deactivate
    self._releasePipelineObjects()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 650, in _releasePipelineObjects
    self.pipeline.releaseQueueForFactoryStream(consumer, consstream)
  File "/usr/lib/pitivi/python/pitivi/pipeline.py", line 747, in releaseQueueForFactoryStream
    raise PipelineError()
pitivi.pipeline.PipelineError
Traceback (most recent call last):
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 160, in _cancelButtonClickedCb
    self._shutDown()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 82, in _shutDown
    self.removeRecordAction()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 193, in removeRecordAction
    self.renderaction.deactivate()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 146, in deactivate
    self._releasePipelineObjects()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 650, in _releasePipelineObjects
    self.pipeline.releaseQueueForFactoryStream(consumer, consstream)
  File "/usr/lib/pitivi/python/pitivi/pipeline.py", line 747, in releaseQueueForFactoryStream
    raise PipelineError()
pitivi.pipeline.PipelineError
Traceback (most recent call last):
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 164, in _deleteEventCb
    self._shutDown()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 82, in _shutDown
    self.removeRecordAction()
  File "/usr/lib/pitivi/python/pitivi/ui/encodingdialog.py", line 193, in removeRecordAction
    self.renderaction.deactivate()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 146, in deactivate
    self._releasePipelineObjects()
  File "/usr/lib/pitivi/python/pitivi/action.py", line 650, in _releasePipelineObjects
    self.pipeline.releaseQueueForFactoryStream(consumer, consstream)
  File "/usr/lib/pitivi/python/pitivi/pipeline.py", line 747, in releaseQueueForFactoryStream
    raise PipelineError()
pitivi.pipeline.PipelineError