I know a boat captain who dove Christmas Island around Xmas 2002, and he said the diving was the best he'd ever done anywhere in the world. Shoals of fish and pelagics, untouched reefs, and very little population pressure. He said that the one dive shop was closing down though b/c of very slow business, which I believe is due to the little traffic the island gets. Aloha Airlines flies charters there once a week and often the plane is mostly empty. The few passengers in it are fishermen who pay lots of money to fish some of the most unspoiled waters in the world.
I've been obsessed with going to Christmas Island after reading and re-reading Thurston Clarke's "Equator: A Journey", where the author travels eastward along the equator starting in South America. One of his last stops is Christmas Island, and he does a nice job at mixing history, the clash of island culture with the outside world, and descriptions of the innate beauty of the island. I highly recommend this book before you go. It's out of print, but it's a classic among followers of travel writing and you can still get copies on Amazon.com.
I almost made it there two years ago. Planning the trip is difficult and expensive -- you often have to go through one of a few travel agencies that have connections there. Visas you get at the airport in Honolulu before you board the flight. Christmas Island is one day ahead of Honolulu, even though they're both roughly on the same longitude, and it lies three hours by jet directly south of Honolulu.
If you go, make sure to post a trip report so others like me can live vicariously through someone who is so fortunate to have made it there. Good luck with your trip.