Just updated to a new dev build of the craft bukkit server and still getting the same error. I really don't know why, so I'm going to reboot the PC. you never know... :D
Because I wasn't sure if the warps might end up inside walls or above fatal drops - once we've got actual frameworks in place we can create proper permanent ones.
(There's also a welcome/entrance one somewhere, but don't need to know that - got enough to work from with these two.)