Sony has announced via Twitter that PSN will be down for maintenance this Thursday or after midnight tonight.
“There will be a routine PSN maintenance on Thursday between 02:00 and 08:00 UK time. PS Network services may be intermittent for some users,” says the official twitter feed.
We would imagine, that as usual, during this period the PSN will be unavailable, meaning you will not be able to do access the PlayStation Store, Account Registration and Account Management.
If you are already signed in then you should be able to continue playing.