hey digital djigit,funny you should mention that,it came up in conversation the other day...
it seems that sitting like that is like rolling your tongue,you can either do it or you can't
in any given room (nb-anecdotal evidence based on me and my mates) about half the people will be able to sit like that indefinitely without making any effort to balance,while the other half won't be able to balance and will just fall backwards...
thing is,it didnt seem to be related to weight or size (although i'd imagine really fat people would have difficulty with it no matter what)