If you like the idea of the iPad's slim "smart cover" and would like that same convenience on your iPhone, Cult of Mac has a clever solution that requires a simple case and a jailbreak tweak.
Cult of Mac uses the SurfacePad for iPhone (pictured above), which is a case designed to act like the iPad's smart cover — only without the handy "automatic unlock" feature. So, in order to duplicate it, they added the Always On Proximity tweak from Cydia, which turns the phone on when something isn't in proximity of the screen — like your case's cover. So, when the cover is closed, your phone stays off, but when you flip the cover open, your screen automatically turns back on.
It's a clever little duplication of the iPad's smart cover feature, and if you didn't want to buy the SurfacePad, you could probably DIY something very similar yourself (or apply it to any other folding case). Hit the link for more info.