Anyone looking to download any new DLC for their games better do it tonight, because as of 10AM tomorrow (EDT), the PlayStation Network will be down for maintenance. According to PlayStation's online blog, you may receive a message that states "PlayStation Network is undergoing maintenance" when attempting to access the PlayStation Store or PlayStation Home.
Thankfully, online play and apps like Netflix and Amazon Instant VIdeo will not (supposedly) be affected, as long as you've signed into the network within the last 5 days. Maintenance should be complete around 2:00 AM Friday morning, but just in case, you can follow this twitter account to receive a notice when the network is restored.