While talking with an EA rep, IGN learned that the GameCube and GBA versions of Harry Potter and the Sorcerer's Stone would be connectable via the link cable.

When you link them up, the Forbidden Forest level on the GBA version will be unlocked. As an added bonus, once you finish the Forbidden Forest level on the GBA, you can play the Forbidden Forest level on the GameCube version.

Both versions should be released sometime this November.

Source: Pocket.ign.com