The American (Merriam Webster) and Oxford English Dictionary entries for preventive and preventative were mentioned in my post of 27 Jun 03, indicating both have been in use since at least the 17th century.
Since these are the two major dictionaries for their respective countries, their acceptance of both makes the question a matter of preference. The OED says preventive is "preferable," but Webster's Collegiate makes no comment. Certainly "preventive" is more common at this time.

Preventative is often used by medical men with reference to a preventative drug or preventative treatment, but others will leave out the extra syllable. I doubt that there is a clear preference among medical practitioners at this time.

Examples, preventative:
Earl Orrery, 1676- "All preventative thoughts of hostility were silenced."
De Foe (on the Plague), 1722- "To send a Preventative Medicine to the Father of the child."
J. Pratt, 1775- "Without meeting any new preventative in my way..."
Pennant (medical), 1774- "The practice of Bleeding- as a preventative against the pleurisy."
Dylan Thomas, 1934- "Do you believe in preventatives (referring to contraception).

Examples, preventive:
Milton (Church directives), 1641- "A preventive fear in case the omitting of this duty..."
Fuller, 1639- "A preventive war..."
Sir T. Browne (medical ref.), 1646- "Physicke is either curative or preventive."
Godfrey (medical ref.), 1676- " Yet would I not have you think there are no Preventives, or means to preserve Health for the future."
New Statesman, 1963- "Released from preventive detention..."

And many more examples of each, with many nuances of meaning.