This is as stupid a question as "Is Yoshi returning?" Yes, yes he is.
Not only is Marth the key character of Fire Emblem, he's the mascot of the series in a sense, even with different lords appearing. Anything else? His castle has already been shown in the Brawl trailer. (Those of you saying it's from Path of Radiance, need I remind you that the Daein castle in PoR was surrounded by SNOW, this one isn't.)
Also, without Marth, Fire Emblem would have still been a shadow game in the US, known only in Japan. Yes, Roy was in there too, but he was just advertising for another japanese game, Marth however was in as the main lord of Fire Emblem. Also, if Sigurd was gonna get in, he would've gotten in melee, as his FE game was before Melee's release, and as the melee poll shows, Marth was much more wanted and popular.
Repeated again, he's not going anywhere.
99%
Nominate Peach x 5