Here is the wording of the Humanoid Shape invocation:
You can assume any humanoid form, as the change shape special ability (MM 306). You can remain in the form for up to 24 hours or until you take a standard action to assume a new form or return to your natural form.
This means, the Dragonfire Adept can turn into any creature that is of type humanoid. I have always thought the Aasimar to be an Outsider, meaning it was an illegal shape for the invocation. However, I have recently discovered that the Aasimar in Forgotten Realms are of type Natural Humanoid, subtype Planetouched.
Does this mean the Aasimar is a legal race for the Humanoid Shape invocation if the game is based in FR?