Thanks everyone. Yes, its very interesting that the EU and UK have higher standards than us...
Technically speaking, based solely on the cabin crew ratio, the 1:50 seat ratio is
lower than the 1:36 passenger ratio (i.e. the operator requires fewer cabin crew), which is exactly what you indicate Virgin Blue has requested. The 1:50 seat ratio is in use pretty much everywhere in the world, with the notable exceptions of Australia (1:36 passengers) and Canada (1:40 passengers). That's not to say that one ratio is necessarily better than another, just that they are different.
The design standards for aeroplanes are harmonized internationally and each manufacturer must demonstrate that the aircraft can be successfully evacuated using the minimum required cabin crew, maximum number of passengers and only half the aeroplane exits. The basic requirements can be reviewed at the following links (these are to the US FAA regulatory requirements, but they are harmonized with the EASA (European), TCCA (Canadian) and ANAC (Brazilian) requirements):
25.803 Emergency evacuation
Part 25 Appendix J --Emergency Evacuation
AC 25.803-1A Emergency Evacuation Demonstrations (Although in draft, the guidance contained in this version of the AC is now typically permitted by the regulatory authorities.)
While there are age and gender requirements for the participants of the evacuations, they are all able-bodied. As well, many aeroplanes are now certified based on analysis rather than an actual demonstration (the A320 is one such aeroplane).