These caches are called throwdowns and there are a couple of interesting links here:
http://www.cacheopedia.com/wiki/Throw-Down_Cache
http://support.groundspeak.com/index.php?pg=kb.page&id=427
A couple of our local cachers tried this trick with some of my first caches and I refused to allow their finds. It is one of the reasons I am reluctant to have spoiler photos. With a spoiler photo you can be 99.9% certain that a cache has gone and therefore may be more inclined to throw a cache down, if you are not so sure you probably won’t.
It should only ever happen with the owners prior permission but some try to put pressure on an owner after they have done it to allow a find. They haven’t found it – they placed it!
