This is a really tough call, book now or book later. I opted to book sooner than later for our October 2009 cruise and it was just because I had an NCL Cruise reward that I had to use or lose.
I found the 2009 version of the Halloween Jewel repo cruise Kuki took in 2008. When we first booked it we were looking at around $5400 for the four of us in an AF minisuite. Since the initial booking they have run three major sales and we now have the price down to $4000 and a $300 OBC. Which is quite surprising because NCL is one of those cruiseines who in the past has refused to honor price drops. I was pleasantly surprised that they changed their policies during these recent sales. Their strategy must be working as this sailing is filling up very quickly. We already have well over 130 people on our roll call for an October 2009 sailing!
I guess one ought to approach this with the same mindset as booking a guarantee, book the price and cabin you're happy with and can afford, if it doesn't feel like a good deal to you, then don't book it. If it is a cruise you have your heart set on, perhaps you ought to book it while cabin selection is good. It is definitely a gamble.