Setting workspace to SDE feature dataset

I have a python add-in that successfully edits data in a file .gdb (with or without invoking the editor in the script), but I can't edit data in an SDE feature dataset. I can edit the SDE data (9.3) manually in ArcMap, and the add-in successfully interacts with the SDE data using search cursors. SP1 is installed. Any suggestions about what might be causing the error in response to this code?

Traceback (most recent call last):
File "C:\Users\kalley\AppData\Local\ESRI\Desktop10.1\AssemblyCache\{87D25F75-9A62-5BC9-A006-4B496936EC0F}\ShieldToolTest_addin.py", line 79, in onLine
edit = arcpy.da.Editor(workspace)
RuntimeError: cannot open workspace

I can provide the full code if necessary...just wanted to keep it simple first.