The port was announced at E3 2010 as part of the platform's official debut. The door is still open for MGS3 to be a 3DS launch title, however, as the new handheld from Nintendo is scheduled to be released before March 31, 2011.
It will be interesting to see how MGS3 controls on the 3DS, which has been fitted with an analog disc and/or nub (technical term). The absence of a second analog nub might make controlling the camera a bit difficult, but it worked out okay for Peace Walker after a bit of playtime.
Metal Gear Solid 3 is my favorite entry in the series so this might just be the game that gets me to buy a 3DS. It will all depend on new features, however, as my PS2 copy of MGS3 plays just fine on my superior, fully backwards compatible 60GB PlayStation 3.