I discovered this a long time ago, but never tried because using Photoshop was a bit painful and complex to obtain good result. Recently, I read on RCGroups that a website was proposing to do the modifications online, with very few parameters, and the ability to save the result on disk.
The website is http://tiltshiftmaker.com
The results depends greatly of the original picture, the angle of the shot, colors, details, etc ...
More information about Miniature Faking on Wikipedia.