Karne, the restriction is season-based only, at least for Aust Nats. A competitor may choose season by season whether to skate adult or standard, but may not skate in both divisions within the same season.
I just double checked my copy of the 2011 announcement for Aust Nats, and competitors in adult divisions were not permitted to have skated in any standard track events after 1 Jul 11. Then this year (2012), the date limitation was moved to 1 Jul 12. Interestingly, the announcement for 2010 Nats listed that limitation date as 1 Jul 09 (not the expected 1 Jul 10). 2010 Nats was the first Nats including adults, so I guess TPTB have tweaked the rule in the meantime.
Isk8NYC - normally you'd be right, but this is ISA we're talking about here! I just rechecked the rule book but there is no mention I could find of eligibility to compete adult, only of permitted or required elements per adult level. There is a separate tech reg specifying required test levels, but it does not address standard vs adult eligibility.