Yup, I've been asked for permission (apparently people don't quite believe the GFDL)
firstly there is the issue that i don't think you can included a gfdl image in a non gfdl book (or at least if you can its not too clear)
secondly there is the fact its nice to ask even if you don't have to and it reduces the risks of cases where people released thier images without realising what they were doing and get angry and/or sending legal threats.