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I have to check .... Is it done through iCloud ?
Maybe you have to enable Push to activate this feature.
 
Has anyone actually seen this work? I dismiss notifications routinely on my iPhone and they're still present in the "All" tab in notification center on my iPad.

The feature is listed on the official iOS feature page: http://support.apple.com/kb/DL1682?viewlocale=en_US&locale=en_US
I am having the same problem. I noticed today that a bunch of dismissed iPhone notifications were still showing up in my iPad Notification Center.

If it matters, I'm using an iPhone 5 and an iPad 4.
 
Works for me - iMessages, Gmail, Calendar alerts work across the system.

Others, which require a separate action on each device, do not - AppStore updates, Cydia updates
 
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