Is teleporting a creature considered to be forced movement of said creature? I have a player interested in a power that adds damage on forced movement and teleport.
No. From the compendium:
Movement that a creature is compelled to do, specifically a pull, a push, or a slide. A creature can be moved in other ways, such as through teleportation, but only pulls, pushes, and slides are technically forced movement.
It's not forced movement, but per the Player's Handbook errata, trying to teleport creatures into dangerous terrain or to a spot where they will fall gives them an immediate saving throw to avoid the teleport.
Page 286: Replace the text for the Destination entry and the Immobilized entry. This change addresses what happens when a creature is forced to teleport, and it clarifies that the Immobilized entry includes restrained as well.
✦ Destination: Your destination must be a space you can occupy without squeezing. If arriving in the destination space would cause the target to fall or if that space is hindering terrain, the target can make a saving throw. On a save, the teleportation is negated.