The Playstation Blog provided an update on the store downtime :
"Hi, everyone! Just wanted to provide an update about the PlayStation Store. The store for both the PS3 and PC will be unavailable starting early morning Monday, April 14 through early morning of Tuesday, April 15. You will not be able to access either store during this maintenance window.
As you know, we are preparing to release the revamped interface of the PlayStation Store for the PS3, which will be available on Tuesday. Regular weekly updates of the store will resume on Thursday, April 17, along with the release of GT5 Prologue.
Thanks for all the positive feedback about the upcoming store design!"