UPDATE: E! Online had a typo that I copied over but is now fixed below. It is the confirmation we will learn what the island is from Jacob.
Here is a break down of spoilers and tidbits via Nasrani at E! Online
1) Episode 6×15 Across the Sea – No series regulars will appear (Confirms what many might of suspected based on the filming for Year 23 with MiB and Jacob)
2) Rebecca Mader aka Charlotte will return (I believe from previous spoilers she returns in the flash-sideways of Sawyer)
3) Episode 6×09 Ab Aeterno:
– We will learn what the island is from Jacob. Last week she said that it is a 4-letter word with out an A or E but this week she says it has an O.
– We learn why Richard does not age