It seems that the USB use flag needs to be disabled for CUPS 1.5.2 in order to print to an USB printer see http://forums.gentoo.org/viewtopic-t-921948.html Reproducible: Always
I'd say it is invalid. The preference of libusb over usblp started in CUPS-1.4, and is already present in CUPS-1.4.x ebuilds.
I had the same problem. May I suggest that you read the ebuild file. In IUSE: you'll find usb! Gerard.
The ebuild actually prints out fat warning... eerror "Your usb printers will be managed via libusb. In this case, " eerror "${P} requires the USB_PRINTER support disabled." eerror "Please disable it:" eerror " CONFIG_USB_PRINTER=n" eerror "in /usr/src/linux/.config or" eerror " Device Drivers --->" eerror " USB support --->" eerror " [ ] USB Printer support" eerror "Alternatively, just disable the usb useflag for cups (your printer will still work)."