With the default permissions in Fedora 11 VirtualBox is unable to connect any USB devices to guests (the devices show up in the list but are all grayed out). It appears this is because all the /proc/bus/usb files are owned by root and inaccessible. Doing a chgrp/chmod of the files to be owned by vboxusers group and readable/writable by the group seems to allow it to work. Unmounting /proc/bus/usb entirely also seems to allow it to work. (It seems that it's mounted by /etc/rc.sysinit.)
Apparently VirtualBox uses /proc/bus/usb (which is deprecated) if it's available, but doesn't check if the permissions actually allow it to be used. It seems the newer /dev/bus/usb is only used if /proc/bus/usb is entirely unmounted. Seems like it should be the other way around and /dev/bus/usb should be used unless it's not present.