UPDATE: Here's D3 Publisher's official comment on the matter:
The game is only announced for Japan. The latest trailer is shown here and [the game] will include 4-player online co-op plus some additional Vita specific features.
So nothing's confirmed for North America. Bummer. Why don't you take to Twitter and voice your love for EDF in the hopes that your voices encourage a localized release. I mean, let's not forget that Tokyo Jungle is coming out in the States. If that can happen, there must be hope for Earth Defense Forces 3: Portable.
ORIGINAL POST: The PlayStation Vita is about to become a home for gaming's greatest alien giant insect-and-robot invasion shooter around, with D3 Publisher confirming plans to release Earth Defense Forces 3: Portable on Sony's handheld (via Andriasang). A new trailer and website confirm plans for a launch in Japan, though there's no word yet of a North American release. We're checking on this, don't you worry.
I'm guessing that Earth Defense Forces 3: Portable (yes-- "Forces" plural, according to the trailer) is some kind of port of Sandlot's Earth Defense Force 3, an Xbox 360-exclusive release that came to North America in 2007 as Earth Defense Force 2017. There's no word on when the game will be released, though the official website makes mention of 2012.
The EDF games are solid budget titles here in the U.S. so I think it's probably reasonable to speculate that we'll see this localized soon enough for North American gamers. That's certainly the hope, because having the ability to squish giant space bugs with rocket launchers on the go would make the Vita exponentially more awesome.