There are other players reportedly involved, and we will flesh out the details as that information becomes available.
Montero, who turned 22 at the end of November, is a C/1B/DH whose value is in his bat, as there are questions about whether he can stay behind the plate. Baseball America had him as the #4 prospect in baseball after the 2010 season, and the #3 prospect in baseball after the 2011 season. After posting an 870 OPS in AAA in 2010, he regressed somewhat statistically in 2011, putting up an 814 OPS. Nevertheless, Montero is considered an elite, top-shelf prospect.
Pineda, who turns 23 next week, was the #16 prospect in baseball according to BA prior to the 2011 season, and was one of the few bright spots in the Mariners' 2011 campaign, putting up a 3.74 ERA in 171 innings for Seattle, while striking out 173 batters and walking just 55.
Rangers fans will recall that the Mariners were close to a deal sending Cliff Lee to the Yankees for a package headlined by Montero in July, 2010. Texas trumped that deal by offering Justin Smoak, who they had previously refused to agree to part with, and the M's ended up sending Lee to Texas, reportedly preferring Smoak to Montero. Obviously, they now have both players.
This deal also makes Prince Fielder to Seattle less likely, since the M's now have young, well-regarded players at both the first base and DH positions (assuming Montero plays one of those positions, rather than catching, for Seattle).
UPDATE -- Enrique Rojas is reporting that Hector Noesi is also going to the Mariners, with Jose Campos going to the Yankees.
Noesi, who turns 25 in two weeks, is a righthander who appeared in 30 games last season for the Yankees, mostly in relief, while posting a 4.47 ERA.
Campos is 19, from Venezuela, and pitched in the Northwest League last season, finishing #3 in BA's NWL top 20 rankings after posting a 2.32 ERA with 85 Ks and 13 walks in 81 innings, and a fastball that apparently makes scouts drool, hitting 95 mph with movement.