http://en.wikipedia.org/wiki/Gold_plates#Returning_the_plates

There are second hand stories, but the primary story from Joseph Smith simply describes the situation as having returned the plates to the same angel who instructed him how to get them in the first place. See the link for further reading.

"Pffftt...Convenient timing! Dum dum dum dum dum!" Yes, of course, good ol' South Park to point out what of course no critic had noticed in the prior 170+ years of my church's existence. We have plenty of reasons to justify why we believe the plates were actual, real objects. Joseph Smith had 11 official witnesses of the plates, not one of whom ever denied having seen the plates in spite of the fact that many became estranged from Smith and the church. Most of those stayed within the movement, often joining some schismatic group, and two returned to the main branch. This singular fact is strong evidence of its authenticity, as those opposed to the church or its leadership had a wonderful opportunity to discredit a key element of the Church's doctrine. Even those who never came back to Mormonism in any form still maintained their original testimony.