I have to say that whilst not a perly solution I use MRBS (Meeting Room Booking System) for this purpose. We use it to schedule time on all our lab equipment including the computers, its quick to set up, easy to modify and looks quite pretty (its PHP/MySQL).

It allows repeated bookings etc etc and has an admin account and user accounts - its what keeps my workplace running smoothly ;)

Bukowski - aka Dan (dcs@black.hole-in-the.net)
"Coffee for the mind, Pizza for the body, Sushi for the soul" -Userfriendly