http://www.karate.org.yu/Matayoshi sai.htm
"At first look, sai looks like kind of a dagger, but actually, its practical application is similar to club use. This weapon has no blade or cutting edge, so it is not intended for slashing but rather for striking. The
top* (saki) is dull, which implies that it was not designed for causing mortal wounds. The main purpose of this weapon is to hurt and repulse an attacker. Above the handle (tsuka), there is a hand guard (yoku) especially shaped for not only protecting the hand, but also for catching the opponent’s weapon and disarming as well."
*saki = the tip of the sai, see diagram in link.
http://www.teako170.com/ddscript4.html
From the Daredevil screenplay:
"Bloody HAND--
razor sharp SAI enter side window.
Prick up against motorman's throat.
Anxious. He turns--very slowly.
Sees a most displeased Elektra.
INSIDE TRAIN - MINUTE LATER
ELEKTRA moves through the cars.
Bloody sai held to motorman's throat."
-edit- forgot commentary.
I won't claim to be an expert, but all the sai training I had many years ago indicated sai have always been blunt tipped, but like the spiked beak on a European poleweapon, they can be used to penetrate armor. Primary swings with a sai tend to be club-like swings. Thrusting is a secondary consideration, and usually done with the "pommel" (tsuka) or "quillons" (yoku), in -for example - a face gouge.
I speculate clubbing somebody with great sai technique makes for a less cinematic movie, one would imagine, so Hollywood and Hong Kong made sai into piercing weapons. Certainly piercing are more dramatic and are far more indicative of a downed foe, as the "blades" pierce the foe. I blame Frank Miller's Daredevil run for further exacerbating this misconception.
In D&D I'd recommend the sai remain a bludgeoning weapon, as that's how they are made and used. I'd also recommend any prospective sai masters to pick up the "Throw Anything" feat from S&F if they really want to glom onto the whole thrown sai schtick. Traditionally, sai were carried in sets of three, so the first one could be thrown and the other two dual-wielded. Despite being bludgeoning weapons, a thrown sai can penetrate a wooden target nicely, primarily because of its mass and balance. (A thrown sai that hits even obliquely is basically all metal - think of it as a large, very heavy, dull shuriken.)
/ramble
Greg