I've been trying to avoid spoilers all season. I do normally watch the teasers though because in the past they haven't ever given enough away to spoil things. However, I felt that Michael being the man on the boat has been spoiled for a long time. The writers/producers gave away the fact that Michael was coming back this season during a panel at some convention (the one where they showed the Orchid video). They made this announcement just moments before begging the viewers to please help others avoid spoilers.
Knowing this makes all other "hints" completely obvious. Ben saying he has a man on the inside. The teaser saying we'll see someone we haven't expected to see. It all pointed to Michael.
Very cool flashforward/flashback interplay there. It had me fooled the whole time until Jin mentioned he had only been married a couple months. There was one clue that, had I been intimately familiar with the Chinese zodiac (or Wikipedia at my fingertips) I would have caught it. The store owner that sold Jin the panda mentioned that it was the year of the dragon...2000 according to Wikipedia. Ji Yeon would have been born in 2005, the year of the rooster.
It's interesting that the grave marker has his date of death as 2004-9-22, the day of the crash. So, yeah, I'm wondering if he's really alive or dead.