There are only a few scanners supporting USB 2 at the moment, the reviews claim the connection speed is nice but don't affect the speed of high rez scans.
USB 2 is backward compatible with USB 1.1 but the slower devices have to be on the end of the chain. Yes, they use the same connectors.

If you want to share a scanner between a few computers, why not get a USB scanner and install the scan drivers on all machines which use it. Some of the scanners are powered by the USB port only and are really portable, so yes, you would be physically sharing the scanner.

BTW any USB device has to support USB2 to support it. There aren't many devices using USB2 a the moment, but that'll improve.