UPDATE: Sony have confirmed that the Playstation Store will not be returning today, with the SCEE blog suggesting that they 'are aiming to have it live by the end of May'.
Sony have stated that from 4pm this evening through to midnight, the PSN will be undergoing 'essential maintenance. In order to game online during this time, players will need to be signed into the PSN before 4pm, otherwise attempting to do so will simply take the player to the site maintenance notification page.
Reports are still out as to whetehr or not the PlayStation Store will be back today as rumoured. [SCEE Blog]