After two hours of frustration trying to pinpoint the reason why the Sydney guide was "lagging" by an hour, I finally telephoned IceTV's support line to get some assistance after the FAQs and forums didn't get me sorted.
Seems the problem is at their end.

The person I spoke with apologised and confirmed that they are pulling the database apart trying to work out why today's guide is screwy. He also stated that tomorrow's
"looks fine" and that it's
"just today" that's gone belly-up.
Hope this heads-up saves someone else from spending a few hours trying to fix a problem that's not actually within their control. Oh - and if you have any programs scheduled for recording based on the IceTV feed, you might want to double-check them!
Cheers
Derek