Lightpeak is not a big deal because it can replace USB. Lightpeak is a big deal because it can replace USB and Firewire and PCI and PCI Express and SATA and SCSI and DisplayPort and DVI and HDMI. It can be used to connect all the components both inside and outside the machine using a single connection architecture.
True when and if LP is fully adopted and widespread among Mac and PC's. Until then, there will be a transitional period with lots of little interface boxes creating connection hubs: LP I/O, then a bunch of Firewire, USB (1/2/3), DVI, DP, HDMI (HDCP compliant), etc. connections. There is a large existing investment in monitors, disc drives, etc., than will need support for some time.
Just like DP computers have dongles for various other display types.
Eddie O