Up to now, there hasn't been any documentation about Slackware version conventions. Usually PV use .0 for major upgrades and .1 and .2 for updates (meaning no major upgrades). In the past, Slackware perform this naming conventions regularly (8.1, 9.0, 9.1, 10.0, 10.1, 10.2, 11.0, and the latest is 12.0). Before PV announced 12.0, many people have discussed about the next version of Slackware. What will it be? 11.1 or 12.0? It's was then 12.0 because it was build on a new toolchain and lots of changes in the packages (mostly became more modular for easy management). Besides that, Slackware 12.0 is the first Slackware release that shipped without kernel 2.4 and have a default 2.6.x kernels. Support for kernel 2.4.x has been dropped, so as for floppy installation.
After 12.0, the next release will be 12.1. Pat has said this (implicitly) by giving a naming scheme on his package (look for it in the Slackware-Current changelog. I'm sure you know what package i'm referring to).