If anyone is interested, it turns out the rumours were true - Lost will effectively be getting a 3 hour finale split over 2 separate weeks (like with Exodus in season 1). The catch is that because of a 2 hour finale for Grey's Anatomy in America airing on the same night Lost was supposed to have it's original (1 hour) finale, the network execs are airing the first hour of the finale as usual, then skipping a week, then airing the second, 2 hour part on the 29th May.
Word is there are so many big reveals the producers wanted to deliver this season that they felt anything less than a full three-hour finale would have cheated the fans. In fact, I'm hearing that the writers' draft for the second half of the finale was 80 pages long, which led all parties involved to realize these stories had to be told now. Anything else would have felt like a rip-off for the fans
So, the schedule in the US:
24th April - Episode 9
1st May - Episode 10
8th May - Episode 11
15th May - Episode 12 (Finale Part 1)
22nd May - Grey's Anatomy finale, no Lost
29th May - Episode 13 (Finale Part 2 and 3)