I don't have release notes available, yet. Your best bet is to take a look at these Blog posts, the very first one, i.e. one at the bottom, in particular:
In short version 3.1 is not a rewrite but a brand new port from one dev environment to another. It was a very sizable effort. The intent was to get WPrinter to a new platform that (a) doesn't have inherited limitations causing newer Windows versions compatibility issues; (b) provides Unicode and 64-bit support; and (c) allows for easier application expansion.
I still have your hiding printers request in the queue. Once 3.1 is released, it'll be much easier to implement new features whereas with 3.0, the application was basically in the bug fixing mode.