Update: 24:00 I can also confirm that Lost's regular timeslot will be 9:00pm on Wednesdays.
There were a number of rumours going around about the Season 5 Premiere being a 2 hour event. I've just got confirmation from ABC that this is infact true.
20:00 - 1 Hr Recap Show
21:00 - Episode 5.01
22:00 - Episode 5.02
Pretty cool huh!