Tim,
Let me try it on my side and see if I can figure out what's going on. I do know that if you make a sproc change you do have to reset the connection otherwise Oracle uses a cached version of the sproc (ie the older signature)...
I would have recommended using variables instead of parametsrs as the next idea but you might have already done this....

Sanjay

Tim, 
I tried out your sproc and it's working without problems...
Here's an example of how getusers is called (this is a sproc that comes with the core) - maybe this will help you tailor your call to the sproc differently..

  Public Overrides Function GetUsers(ByVal PortalId As Integer) As IDataReader
            Dim OracleSqlHelper As New OracleSqlHelper
            Return CType(OracleSqlHelper.ExecuteReader(ConnectionString, DatabaseOwner & ObjectQualifier & "GetUsers", GetNull(PortalId), "ReturnValue"), IDataReader)
        End Function