Nah thats wrong.
The reason for this is because (forgive me if this has been stated already),
In the vast majority of ICU's there are no doctors there 24/7. Yes, certainly there are many hospitals that have intensivists, and hospitalists, and etc... But in even these cases many times the MD isnt 24/7. MOST ICU's in the USA dont have this luxury.
Now...keeping that in mind...the ICU is where the sickest of the sick patients are...period. Couple this with a nurse who hasnt an MD on her hip, the 'autonomy' we're talking about is born of necessity(sp?). Of course, ALL NURSES are bound and dependant on physicians orders....otherwise we'd be practicing medicine...but...the ICU nurses often-times dont have time to wait around trying track down a doctor while working with critically ill patients. If they were to wait for physician direction (which can waste considerable time) these same patients may die.
As far as 'evidence' of autonomy is concerned, i'm sure ICU protocols, ACLS algorythms, etc. have been mentioned. But a better (i think) example of how an ICU nurse can be considered more 'independant' is that in many ICU's (the one i currently work in for example), the 'routine' ICU orders include various labs and tests (cbc, cxray, abg, etc.) that an icu nurse can order "In an urgent situation". This is important precisely because there is no protocol or algorythm attached to it....but rather based on clinical judgment(sp?). And yes the MD will eventually be informed and may even disagree... but it's the nature of the ICU beast.
And as far as being more autonomous than ER nurses...it's simple. ER doc is always there...within earshot. And...any critically sick patient that comes through and is NOT being closely followed by the ER MD is one where patients dont want to be...!! Also, ICU nurses (ideally) only handle critically sick patients...the vast majority of ER patients are not critically sick. So...the ICU nurse becomes a specialist in this regard...because it's the only kind of patient she handles.
Lastly, i think it's important to add that although there is the 'appearance' of having more autonomy, it's simply because of the work ICU does. If the ICU nurse is thought of as a 'specialist', then this pseudo-autonomy can be thought of as one aspect of this specialty. 'critical care nurse' can be applied to many nurses...including ER nurses and ACLS-trained tele nurses, etc... but the critical-care nurses who work in the ICU setting are the 'specialists' i'm speaking of.