Assuming everything goes well, and that the world doesn't end (which is supposed to happen on Saturday), Sony's PSN store could be back up and running Next Tuesday. That date is based on a memo to publishers from Sony. Read the whole report on Eurogamer, lets just hope the planet doesn't fall apart in the meantime.
I'd expect the store to collapse shortly after under the weight of demand from users. The Lazarus-like return will be followed by bi-weekly updates to shovel all the lingering content into place while everyone downloads their free games. Not sure I'd like to be selling a lightweight game in those weeks.