Hey there, time traveller!
This article was published 15/2/2014 (1282 days ago), so information in it may no longer be current.
iPhone: Lytro's shoot now, focus later feature is pretty cool, and if you want something like that on your iPhone, Tadaa SLR is what you need. It's essentially a blur-and-bokeh filter app, but the interface makes it almost foolproof.
Snap a photo or upload one from your camera roll and Tadaa ($1.99 in the iTunes App Store) will ask you to highlight your subject. You need to "paint" your subject with your finger, erasing the parts you don't want and zooming in for more precision. I'd suggest you keep the "Detect Edges" option on, but if you want more control, switch that off.
Once that's done, you can adjust the amount of blur you want and tap on either the highlighted subject or the background to focus on the selected part. There are other settings to tweak, too, like highlights and gloss. When finished, upload it to your Tadaa account or social networks. You can also browse through creations uploaded by other users.
While this app makes the "tap to focus" process very easy, this is just a software hack and won't give you the same kind of effect Lytro does. Still, if you can't afford a Lytro, or you can make your DSLR camera give you a similar effect.