BTW (off topic), what happened to AmigaOS 3.2, 3.3, 3.4, 3.6, 3.7, 3.8?
Perhaps after such a long gap between 3.1 and 3.5, it was thought nessecery to increase the version number a bit more? As for 3.9, IMO it hints that it is the last of the 3.x series - either there wouldn't be anything else, or what would come after would be very different.
Looking at the OS4 presentation slides etc, I think that there will be a 4.1 (although perhaps it will just be for beta testers? :-? )