According to Tom Sloper...
http://www.sloperama.com/mjfaq/mjfaq19.htm
Q: When is a tile down?
A: A discarded tile is "down" when it touches the table OR is
completely named, whichever occurs first. If you pick a tile from the
wall, then touch it to the tabletop, it's "down." You must say its
name and take your hand off it. Likewise, if you say the name of the
tile in full, it's "down." You must put it down and take your hand off
it.
For further reading:
RDWW - p. 51 (rule 53), p. 88
NMJL - p. 18
Q: Can I change my mind after my discard is down?
A: No. When a tile is "down," it is too late for the discarder to take
it back. If you've said its name, you must put it on the tabletop and
take your hand off it.
Q: Darn! I shouldn't have thrown it away! Well, can I call it back and
make a meld or win with it?
A: No. "Down is dead." Well... The tile is dead to you, anyway - as
long as it's not a joker, any other player except its discarder can
call it. Nobody can ever take back a tile she just discarded, in any
way, shape, or form. You can't make up rules to undo a blunder you've
made.
See FAQ 19AM for more "change of heart" rules, and see FAQ 9 for the
Tom Sloper philosophy of how errors should be handled.
For further reading:
RDWW - p. 65 (rule 109), p. 99
NMJL - p. 18