Have to say, lots of great and very interesting views on the whole boarding without cockpit crew issue, its interesting that procedures vary so widely across the industry. I wonder with the introduction of EU ops for european airlines (as opposed to JAR ops) will policies and procedures like this one become more standardised?
So heres my 2 cents worth and how we do it:
The CC are permitted to board passengers without cockpit crew on board provided the aircraft is not being fueled and a full cabin crew compliment is present.
In relation to how we know if fueling has happened or not - just ask the dispatcher - they are responsible for all aspects of the turnaround and should be aware.
Concerning aircraft servicability - in base an engineer is present for the majority of the turnaround and generally are great about informing us of defects or problems that would stop us boarding. Obviously the purser should be able to make an judgement as to whether to hold off. It is rare that we would be downroute and the pilots would not be present, happens the odd time on long haul ops but its rare.
In relation to passenger safety and potention emergencys while on the ground, whilst the cabin crew dont claim to know as much about the aircraft as the cockpit crew we are still trained to operate on our aircraft and are aware of whats normal and what we should be concerned about, eg smoke, fire, strange smells or noises from the aircraft. In the absence of either pilot the purser becomes the most senior crew member onboard and the decision of whether or not to evacuate the cabin rests with him / her.
Please dont jump down my throat for that comment - we are aware of how much more the pilots know about the aircraft than us, the point im making is that we know whats normal and whats not onboard and have the ability to get the passengers off quickly if we feel it is necessary. After all if the pilots ever gave the command to evacuate it would be the cabin crew that carried out the evacuation.
Finally in relation to cockpit security and access - crew must be at their stations for boarding, the purser and usually at least 1 other crew member are always in the area of the fwd galley and as such the cockpit door. No access would be granted to anyone other than an engineer.
Anyway - again im not saying our procedures are right or wrong - but thats just my view on how we overcome some of the problems mentioned in previous posts.
Looking forward to hearing more on this post - its an interesting one!