Again, we haven't seen the total power list, but if the Swordmage continues to remain in this vein throughout its 30 levels I don't foresee it over shadowing any of the other classes. They may have similar roles, but they go about them in very different ways.
The Swordmage is a defender. Let's look at the most comparable classes: the other defenders, Fighter and Paladin. Each class seems to be built around two roles a primary role which is listed in the book explicitly and a secondary role which is implicit in their power list. Further we see major differences based upon their power source.
The Fighter is a martial defender/striker able to deal gobs of damage.
The Paladin is a divine defender/leader able to take and heal gobs of damage.
The Swordmage seems to be an arcane defender/controller able to deal a lot of damage to a lot of enemies.
There is more than a flavor difference between the classes, but rather a mechanical difference. Look at the differing taunts and you'll see this.
The Fighter marks everyone he swings at. EVERYONE he swings at. His challenge allows him to make an opportunity attack against someone who doesn't normally provoke an opportunity attack just by marking him.
The Paladin can call out one target at a medium range and mark him. That target stays marked only so long as the Paladin actively engages him. If that target doesn't go after the Paladin then he suffers divine retribution.
The Swordmage can single out a single nearby target at mark him. Then he can let that guy wander off to a medium range. When that guy inevitably attacks someone else the Swordmage can either teleport to his back and stab him or interpose a shield of arcane force that reduces the damage that the target was planning on doing to one of the Swordmage's friends.
For further illustration about how classes can serve the same role but not overlap just look at the Ranger and the Rogue in the PHB. Both are Martial Striker/Strikers. Both deal gobs of damage without magic. They have a totally different feel and totally different combat situations wherein they excel. As long as the classes being released maintain this kind of diversity of powers and flavor, I don't see anyone's toes getting stepped on.