On the left page to the right of the Photo viewer app screenshot I've embedded below:
http://cdn.jp.playstation.com/tgs2011/pdf/Hyper_p58.pdf
The captured screenshot will be stored alongside any photos you've snapped or transferred over.
The ability to take screenshots anywhere was strongly hinted at before with the ability to share them online via the free downloadable social media apps like twitter and Facebook.
The physical button method of capture allows you to capture the exact moment you want without delays or preventing you from controlling the game as you capture like the PS3's in-game XMB method.