My guess is, if you approach The Talk as "The Talk," you're already setting yourself up for failure. Wouldn't it be easier to tell a kid what a penis is while he's peeing on you during a diaper change? Potty training would be the outside limit for naming the body parts, no?
Then the sex/reproduction part can be part of the baby brother discussion. Or for when your Danish sex book for kids arrives from ebay. In any case, this NYT story is hi-larious[ly informative]. Sex Ed for the Stroller Set [nyt]
Previously: Ausgezeichnet! Where babies come from
Also: Laid-Off Dad had a funny story about his kid showing off his vast genital vocabulary. Can't find it right now, though.