I think it'll be one of the biggest drops ever in Japan. If you actually compare sales to install base its already sold to a pretty huge 25% or so of the install base. It has to drop harder than hell there really unless it continues to drive a prolonged spike in console sales (which I don't think it will do personally despite best hopes of the PS3 fanboys).
US and Others I don't know. Each has a bigger install base and could support a higher level of continued sales for a while.
Whichever way you look at it though this title will soon reach a point where it will need new PS3 owners to see much continued sales vs install base.
I do think it will have long legs though as I think a fair number of future PS3 buyers will always pick it up - it'll be one of the regular purchases vs a game that total disappears IMHO.
Try to be reasonable... its easier than you think...
