We've determined that a call to InsertPIData immediately following an IISReset can cause the web service to hang.  A forthcoming release of the PI SDK in the near term fixes the problem.  In the meantime, the work-around is very simple: make a call, any call, to GetPIArchiveData or GetPISummaryData.  The call performs some necessary initialization and subsequent calls to InsertPIData complete successfully.