Appengine LogService has an undocumented quota: You can do up to 1,000,000 reads from it per day, and then you get the following error:
Traceback (most recent call last): File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/webapp/_webapp25.py", line 701, in __call__ handler.get(*groups) File "/base/data/home/apps/xxx/3.356325783019142341/xxx.py", line 355, in get for request_log in logservice.fetch(start_time=start_time, end_time=end_time, version_ids=["3"]): File "/base/python_runtime/python_lib/versions/1/google/appengine/api/logservice/logservice.py", line 414, in __iter__ self._advance() File "/base/python_runtime/python_lib/versions/1/google/appengine/api/logservice/logservice.py", line 427, in _advance response) File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 94, in MakeSyncCall return stubmap.MakeSyncCall(service, call, request, response) File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 308, in MakeSyncCall rpc.CheckSuccess() File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_rpc.py", line 133, in CheckSuccess raise self.exception OverQuotaError: The API call logservice.Read() required more quota than is available.
In addition, when you reach this quota, you will see the following on the toolbar: (AFAIK you do not see this line before this): 
At this moment, it is not documented at all, and it seems that it is also not paid.
See also: http://groups.google.com/group/google-appengine/browse_thread/thread/61fac55e1a2d521
Hope this saves you some time.
Let me know if you can think of a workaround ... (just to ask a question;))