[RPG] teleport adjacent to a moving enethe, and then take an attack of opportunity

I was recently discussing interesting tricks that combat casters can use to control the battlefield, and the following scenario comes up.

The party is defending something against advancing enemies. There's a significant area to cover, so they're having to move about. One party member is a melee combat character with access to magic (we were talking about a Duskblade, because our party has one).

1. The player readies an action: "when an enemy crosses this line, I will use Dimension Door [or similar] to transport myself to a spot beside them".
2. An enemy runs across the line.
3. The readied action triggers, and the character appears beside the running enemy.
4. The enemy, having not yet finished their move action, keeps running.
5. The running enemy exits a square now threatened by the character.
6. The player takes an attack of opportunity on the running enemy.

Is this legal?