Planned maintenance will pull PlayStation Network offline for about eight hours on June 25. Sony today announced on the PlayStation Blog that the offline period will extend from 9:30 a.m. PDT to about 4:00 p.m.
During that time, PlayStation Store, PlayStation Home, and Account Management will be unavailable. Note that online multiplayer for PS3 and PS Vita will still be accessible if you sign on to PSN between now and the beginning of maintenance (of course, you'll only be able to play with the limited pool of other people who took similar precautions).
Sony plans to put out a new version of PS3 Firmware 4.45 on June 27--the patch adds trophy notification options and general system optimizations, and may even refrain from soft-bricking systems this time.