Two things:
1 - any plan that doesn't put scheduled maintenance into effect is a plan doomed to failure. You can quote me on that, as I've seen it applied to reality more times than I can count.
2 - "five nines," the mythical goal of DR planning, is 99.999% *percieved* uptime. Scheduled downtime is not considered as part of the downtime in calculating your "total nines."
In short, there are no datasystems that can be up 24/7 without proper maintenance windows. They simply don't exist. While there are many ways to eliminate a tremendous amount of uneccessary downtime (with the proper budget), nothing will ever remove the need for proper maintenance on the server level, even mainframes occasionally need maintenance.
Always remember that a single failed disk drive can bring even a redundant cluster crashing to its knees. Reality is a harsh mistress, an no less so in the datacenter than anywhere else.
Mike Talon