MyControl.ClientID returns what you might expect, the ID that the control will have in the client HTML code.
All in all, it's wasn't a major issue. A little string manipulation and the use of MyControl.ClientIDSeparator did the trick. However, I think it is resonable to expect the two values to be equal since they refer to the same control. The fact that they are not seems like a bug.
On a somewhat related note, shouldn't there be a constant for "__EVENTTARGET" somewhere in the .NET API? After all, you're not supposed to assume the client ID of the control or the postback method call. What happens if Microsoft decides to change the ID of the form element some day. It's odd that they didn't seem to take the same approach as they did with the other client values.