PSN will go offline at 4:05pm GMT on Wednesday March 9th for just over 11 hours. Sony have said that you will still be able to play online, provided you login before the maintenance begins. Sony also stated that any trophies collected during the downtime will still be recorded, however they will not be added to your profile until the downtime has finished. It is not known if the US will be affected by anything similar.
Thanks to EU Playstation Blog.