theres no way Marth is getting cut. you know why? because he's the original lord, the only character besides Ike to be in two games, his game is being remade for the DS, the list goes on. he's the only other Fire Emblem character that actually DESERVES to be in, and Sakurai knows that... seriously, what do you think he is? some kind of idiot?
Reasons Marth might be cut:
1) Fire Emblem, as a series, has many installments with almost no character overlap. It is impossible to represent the entire series. Given this, Sakurai might want to represent as many different games as possible. Therefore, instead of just bringing back the same lords, he takes the old ones out and puts some new ones in, to ensure maximum series representment across Smash Bros. as a whole.
2) The most highly renowned Fire Emblem game in Japan is FE4, starring Sigurd. Sigurd was even originally planned to be Marth's clone in Melee instead of Roy (or maybe Marth was the clone...but it doesn't really matter). Sigurd has pretty much the exact same build and weapon as Marth, and neither one of them has anything unique that contributes towards building their moveset. Sakurai might have decided he couldn't have them both in and give them both unique movesets, and therefore chose to put Sigurd in and have him inheret Marth's moveset, Ness/Lucas-style.
3) Marth's moveset was very overpowered (high speed AND power AND range with no disadvantage) and, in my opinion, incredibly uninspired (basically every A attack is a vertical crescent slash in the suitable direction). It might have just been deemed unsalvageable. Or the changes that would have had to be made to it for it to be viable for Brawl would have caused it to be unsuitable for Marth conceptually (this could be the origin of Ike's moveset for all I know...even though their roles are different, the movesets themselves have some resemblance to one another, especially in the specials).
Do any of these things singularly deconfirm Marth? Of course not. The point is it is possible for the developers to have decided to remove him for logical reasons, without being insane or anything. That in combination with his precarious position in the magazine paints a somewhat grim future for Marth.