Depth of field and Tilt-shift effect
Replicating the effect of tilt shift lenses in photography seems to be becoming increasingly popular of late – it’s a nice effect, which will probably become so vastly over-used that it becomes a dull cliché in no time at all (if you don’t think it has already), not least because it is so incredibly easy.

Image ‘borrowed’ from here, who obviously ‘borrowed’ it themselves…
In the mean time, here is a slightly more involved technique utilising much more complex masking than most tutorials out there:
Faking Tilt – Shift | Martybugs.net [new window]
CS5′s new 3D tools open all sorts of interesting possibilities for creating other realistic depth of field effects. An interesting short video tutorial showing how these tools permit you to do this without using masks or additional blur filters can be found here:
Creating Depth of Field | Planet Photoshop [new window]