easyjet

When I tried a dummy booking just now for Paris to Pisa it told me that the route was for sale until March 26, 2011. Either they are discontinuing that route then (March 26 corresponds to their switch from "winter" flight schedule to "summer" schedule) or you are just a couple of months too early to book it.

Just checked another route for May 2011, and found a page that said that their schedules are loaded seasonally, twice a year. No telling when they will be loading the summer flights, so I would suggest you just keep checking back as March approaches. You can also get on their e-newsletter list.