It's taken some time, but finally CUPS-1.4, a "new" major release of "the standards-based, open source printing system developed by Apple Inc. for Mac OS® X and other UNIX®-like operating systems", will be arriving for our stable users in the near future. The arch teams are testing, see bug 333781 for the details and progress.
So what does that mean? Well, technically there are a lot of new features since CUPS-1.3. In practice, I hope that some of the ancient Gentoo bugs like #341127 or #349496 go away now, since finally CUPS supports some of the features that modern desktop environments require. Also, right now CUPS is an embarrassing security hole, which will also be fixed by the upgrade.
In the version about to be stabilized, we do not support the new libusb-based interface for USB printers yet. Accessing USB printers will be done via the USB printer functionality in the kernel, as in CUPS-1.3. Why? Simply because there are still way too many problems with it. If you want to give it a try anyway, just keyword net-print/cups-1.4.6-r21 - the only difference between -r2 (the stable candidate) and -r21 is that -r21 supports the libusb backend with the usb use-flag. Cheers!