Though Junichi Tazawa might be the first consensus NPB first round draft pick-caliber player to jump directly to MLB, he's not the first to have considered it. And even when he signs, he won't be the first amateur out of Japan to sign with an MLB organization.
The guys listed below have all either negotiated or signed with MLB teams before playing pro ball in Japan. None of the players listed elicited the kind of reaction Tazawa got, but none of them pursued an MLB deal with the same level of fanfare that Tazawa has now.
Koji Uehara: came close to signing with the Angels out of college, but backed off because of language challenges and having to start in the minors. Went on to have immediate success in NPB, and subsequently make numerous requests to be posted. Finally coming to MLB this off-season.
Hayato Terahara: taken directly from Gary Garland's excellent site:
Dodgers V.P. Tommy Lasorda personally tried to sign the 18 year old high school phenom with a 98mph fastball, Hayato Terehara, laying on the blather very thick as only Lasorda can. Terahara ultimately decided to remain in Japan and was drafted by the Daiei Hawks after a lottery drawing between the Hawks and three other Japanese teams.
I was in Japan when this happened and while it was reported in the media, Terahara didn't really seem interested in signing with the Dodgers. Terahara spent several ineffective years with the Hawks, then got traded to Yokohama where he immediately blossomed into a frontline pitcher.
GG Sato: signed with the Phillies after college and played a couple of years in their system. Drafted by Seibu afterward with a late round pick and eventually became a pretty good player. Kind of a late bloomer.
Kazuhito Tadano: went undrafted in NPB because of his appearance in an adult film while he was in college, but the Indians were willing to give him a contract. Tadano pitched briefly in the show but never really did well enough in AAA to get an extended shot in the majors. Nippon Ham drafted him with their first pick in 2007, and he's back in Japan now.
Sho Nakata: drew interest from the Twins, and the Mariners reportedly had a $3M offer ready for him (can't find the link now). Chose to enter into the NPB draft and was selected by Nippon Ham. Just wrapped up his first year with the Fighters' farm team.
Robert Boothe: Grew up in Japan with an American father and Japanese mother. Boothe pitched in college but didn't have must statistical success. Still, the five NPB teams that were said to be interested in drafting him backed off when he decided to sign with the Dodgers.
There's a number of other lesser-known Japanese-born players playing affiliated ball in the US. JapaneseBallPlayers.com has a pretty comprehensive list of guys that are currenlty on US minor league rosters, as well as some notable former players.