Is it possible to automatically turn vibrate mode off at night? I usually have my iPhone in "vibrate mode" (ringtone off, vibrate on). However, I'd like to additionally have it turn vibrate mode off automatically between for example 11 PM and 7 AM because I don't want to be awakened (or have my wife woken up) by some spam mail that arrives at 2 AM.

Some apps allow to define periods where notifications are suppressed, but it seems the Notification center (or at least the Mail app) does not. Is there any way to achieve this (on a non-jailbroken iPhone)? 

I know I can turn it off manually in Preferences, but that's cumbersome, and I'm going to forget either to turn it off or to turn it back on.
EDIT:
This is a feature in iOS 6. You can schedule notification suppression by opening the Settings app &gt; Notifications &gt; Do not disturb.
Original Post:
Unfortunately, there is no native way to do that in iOS 5.
Like you thought, the notification center, currently does not allow customization in that way. I think Apple could have added features similar to that one, but have chosen not to, either because they haven't wanted to take the time (but that hasn't seemed to stop them in the past), or because they have a big focus on keeping things simple.