As reported on the PS Blog, the PSN will have maintenance on Thursday. However, you will not be able to use the PSN period. No online gaming, no nothing. And you cannot use sites that use PSN Authentication, such as the PS Blog. You will be signed out if you are signed in when the servers go down. Maintenance is expected to start at just after 8:00 am Pacific and will last for several hours, until approximately 10pm Pacific.
However, you should stay tuned to their Twitter feed for updates.
So, what are you going to do while the servers are down?