I have the same problem specified in this thread:
The posted workaround doesn't work. Yes, I have the function setup in my child theme, have specified the parameter with a differently named editor stylesheet. No luck. Twenty Ten's editor-style.css is still loaded.
This is listed as a fixed bug here, but again I'm doing what's specified in the bug description as a workaround and it's not working.
(Note: I'm using the TinyMCE Advanced plugin.)
Any ideas as to what I'm missing?
UPDATE: False alarm! The workaround is working.