A previous suggest the island becomes lost. This episode reveals that the island needs to be "moved". I believe Penny shows up on the island looking for Desmond and, out of range, Desmond sees the island vanish just as he receives a radio transmission from Penny.

Perhaps then he escapes somehow and then finds Charles Widmore. After some exposition they form an uneasy partnership with a common goal. Desmond is now pitted against former allies as the losties are now choosing sides.