I should add some verification to the New iRule dialog to disallow this. There are server based iRules (look at the iRule list in the admin gui) that are not intended to be edited and they all start with underscores. We chose to not expose them in the editor and the only way to determine if they are system iRules, is by the beginning underscore. At this point, the Editor does not support iRules beginning with underscores. I'll see if there is a way I can work around this but for the mean time, this is why you don't see them listed in the editor.
-Joe
Nimbostratus