MobileMe launching tomorrow evening
Apple is getting ready to launch their new MobileMe service, an upgrade to their .Mac service, July 9th beginning at 6pm.
At that time .Mac will go offline and will not be accessible while the upgrade/migration is taking place and being tested with the only exception being .Mac email which will continue to be available.
They are not sure exactly how long after 6pm the new MobileMe service will be live but their maintenance window is scheduled to end at midnight so by then is a good guess.
For more info about MobileMe check out the product page here.