I've written a daemon which connects the ALSA sequencer with the usbdevfs device file of a USB MIDI device. You can get it at http://www.informatik.uni-halle.de/\~ladischc/usbmidid.html
It requires a kernel with USB and usbdevfs support (obviously), and with hotplugging support and hotplugging scripts installed correctly.
It should work with most USB MIDI devices, but currently it has been tested with Roland SC-8820 only. I don't have the slightest idea whether the code for parsing the device descriptors of fully compliant devices woks at all. :-) So, please tell me if it works for you.
Retrieved from "http://alsa.opensrc.org/USB_MIDI_sequencer_support"