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.
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.