i'm a firm believer in "the ending is how you interpret it yourself, so the ending may be different for different people..." Having said that ...
Not that you would have any way of knowing it from the context of the episode, ABC released a statement that the end scene of the plane sitting there was not part of the episode narrative, but rather something ABC/network had put in order to allow viewers to 'decompress' from the events that just unfolded before jumping to the next show/news/late show/etc.
Link to story here:
ABC clarifies that everyone on LOST was NOT dead the whole show | SCI FI Wire
That helps. Given how many people came to similar conclusions, the episode could have been more clear on that.
This summary from the replies on the article was pretty succinct:
"Jack's conversation with his dad at the end pretty much explained the whole "dead" thing. Everyone dies. They just don't die at the same time. Hurley and Ben may have been guardians of the island for a 1,000 years. Kate, Sawyer, Richard, Claire, etc. may have led full, rich lives after escaping the island. The church was just the focal point for drawing all of their spirits back to the same place and time so they could move on together."
Since there was a lot of wierd stuff happening, Christian's quote didn't have to mean that he was referring to actual life, so much as "existing in any plane".
The real problem was that unlike the first escape from the island, the show ended before it locked in the reality. Seeing the plane land with Richard on it, or similar Oceanic 6 reaction would have helped lock that in that it really happened.