I've written a few posts about restarting USB service in Ubuntu before. This is an update to that process for Ubuntu 9.04 (Jaunty) and Ubuntu 9.10 (Karmic).
As of Ubuntu Jaunty, the usb kernel module is called usb_storage. To restart USB you need to remove this module and add it back in. This can be achieved with the following command line one-liner.
This is useful if you've been having trouble getting a USB drive or digital audio player to mount.