If you POST a new Repository JSON to Pulp's REST API and then attempt to sync the API with the CLI or API you get:

011-01-18 16:55:45,485 [ERROR][Thread-27] failed() @ task.py:207 - Task id:d87a44ab-2366-11e0-a518-0019d1630404, method_name:_sync:
Traceback (most recent call last):
  File "/home/mmccune/devel/pulp/src/pulp/server/tasking/task.py", line 131, in run
    result = self.callable(*self.args, **self.kwargs)
  File "/home/mmccune/devel/pulp/src/pulp/server/api/repo.py", line 1170, in _sync
    synchronizer)
  File "/home/mmccune/devel/pulp/src/pulp/server/api/repo_sync.py", line 93, in sync
    repo_dir = synchronizer.sync(repo, repo_source, skip_dict, progress_callback)
  File "/home/mmccune/devel/pulp/src/pulp/server/api/repo_sync.py", line 385, in sync
    report = self.yum_repo_grinder.fetchYumRepo(store_path, callback=progress_callback)
  File "/home/mmccune/devel/grinder/src/grinder/RepoFetch.py", line 294, in fetchYumRepo
    if not self.skip.has_key('packages') or self.skip['packages'] != 1:
AttributeError: 'NoneType' object has no attribute 'has_key'

I think the issue is the CLI specifies the skip property but my API call doesn't.