To quote Clark Gregg (who plays Agent Coulson):
What’s the deal with Tahiti. Tahiti’s a magical place. I keep saying that… it’s where I went to recover after being impaled and as I say in the pilot, it was a spectacular experience, I had a beautiful physical therapist and I read Travis McGee novels all day.
But every time someone mentions it lately I seem to go into post traumatic stress so I think there’s a little more to Tahiti than meets the eye.
There’s a much darker, more interesting story there. Something much more magical has gone on to bring this guy back to life.
Because of the word "magical" one theory is that Scarlet Witch had something to do with his resurrection. (Scarlet Witch will appear in 'The Avengers 2'). But at the moment it's pure speculation.
In an interview (Nov 18, 2013) producer Jed Whedon had this to say about Coulson's resurrection:
We know that there's going to be a lot of theories and there's no chance that whatever we reveal won't, at some point, be written on the internet. We're just trying to come up with ways to make it surprising or make it not how they expected it to unravel.
We know what it is, but we know that no one else does yet.
The last sentence is interesting, because it implies that the most popular theories, which Jed Whedon must be aware of, are probably wrong.