The PlayStation Store will be closed for maintenance beginning early morning Monday, April 14 through early morning of Tuesday, April 15.
That will give you some time to do your taxes and Sony some time to finish up the re-vamp and re-awesome of their storefront.
On the 17th, Gran Turismo 5 Prologue will be ready for download and purchase. You should get it. Seriously. It's going to be worth it.
PlayStation Blog: PlayStation Store Update