I have tried this with the current build (1.8.3) on both XP and Windows 7 and the control behaves as it should: When the the TaskDefinition.Settings.ExecutionTimeLimit is set to TimeSpan.Zero, the checkbox is unchecked and the combobox is disabled. Please check your version.
On JoeyChaps earlier post, I want to make a comment for other readers: You cannot set both the Principal.GroupId and Principal.UserId properties. You use GroupId if the account is a group and UserId if an individual identity. Also, GroupId is not supported under XP (1.0).