Browsing the Gentoo forums I stumbled on an odd problem with one of these "easy-but-who-the-hell-would-have-thought-of-this" nasty solutions.
Symptoms: You connect a USB hardware (e.g. an USB scanner) and nothing happens. It seems it is not recognized. But you know that your scanner/printer/whatever works and it should work with Linux! (This often happens with SiS motherboards)
Solution: Check you have OHCI/UHCI support enabled. If not, enable it. Try
modprobe ohci-hcd and/or
modprobe uhci-hcd as root. If even this doesn't work, chances are you compiled your kernel by yourself (most distros should include OHCI): check in your kernel config you have OHCI enabled.
Rationale: The problem is that a USB 2.0 controller card is often coupled to OHCI (or sometimes UHCI) controllers, to deal with low or full speed devices, while the EHCI controller keeps the high speed devices for himself. The hardware switches controllers depending on the attached device. So if you only have EHCI drivers and your scanner switches to the OHCI/UHCI controller, there won't be a driver for that controller -and therefore, your PC cannot communicate with what's attached there. You can read the full explanation here, look for "USB 1.1. "Companion Controllers"".