I have done some of this... one small example I recall was providing a strange device - no plans for it, or any idea what it could be used for, but for use as a potential plot tool later... or a door stop for the PC's if not.
Something I did in one campaign was have the PC's spot someone watching them a few times during the campaign (three times I think)... they would look for the figure, but find nothing... I had no plans for who that was, or why he was watching the players at the time, but I thought that I might have some cool idea later that could tie back to this mysterious figure. I did end up using him as a spy for an NPC they later met - not particularly dangerous or anything, just someone gathering info on them.
Short answer, yes absolutely!
---edit---
Oh, duh! The best one I used was having a figure escape out of the back of a cavern the players came into at level 3 or so. Between them and this figure were a number of kobolds who immediately attacked. the PC's had no idea who the figure was, or why he/she would be in a room with kobolds...
...so at the end of the campaign (where some time travel was involved) they were told they needed to place this artifact that had been carrying around into a pile of treasure so as to make sure everything was set back to normal. They knew they should not be seen and entered into a series of caves... when they came to the location, one of the players disguised himself and snuck up to the treasure pile... nearby were kobolds and on the other side, he saw himself with the group from many many levels back, at which point they realized they had come back to return the item so they could find it again.
I had no plans in particular for that figure until I thought of how to use him there near the end of the campaign.
Anyway, fun stuff
