The maintenance tomorrow will run from 3:00 a.m. PDT / 6:00 a.m. EDT until 11:00 a.m. PDT / 2:00 p.m. EDT.
Since WoW will be down, I recommend checking out Champions Online if you haven't already. It's a nice comic book MMO that has some real potential. One thing I really enjoy about it is that its graphics are not super amazing -- they look like comic book graphics. In that sense, it's a very immersive graphical experience.
Kyle Horner of our sister site Massively wrote the WoW player's Guide to Champions Online. Check it out for more information about a nice game you can play during WoW's downtime.