The PlayStation network will be down for maintenance tonight between 9:00PM and 2:00AM. The announcement was made on the SCEE forums but the post did not state why maintenance was being carried out. We speculate that the network is being prepared for the upcoming PSP and PS3 firmware updates, which are said to be released soon. During the downtime, you will not be able to sign-in to the network.
However, if you sign-on before, you will have no problems playing online. As you may have read, the PSP will be getting access to the PlayStation Store in firmware v5.00, and new features will be added to the PS3 PlayStation Store.