Flour which is wet for 18 or more minutes is considered to be "leavened", and therefore chametz. However, though there's some disagreement, this restriction is generally not applied to matzo, or food items made from matzo meal (e.g., matzo ball soup).
Wikipedia tells me that this is because:
...matzo and its derivatives are neither "leavened" nor "leavenable" and therefore are permissible for consumption during Passover.
Matzo meal works reasonably well as a flour substitute in some recipes (though see linked question below about whether it would work for traditional leavened bread).
Since matzo meal is apparently considered not to be leavenable, would a traditional, leavened bread made using matzo meal in place of flour be considered kosher for Passover? If not, why not (and why doesn't the same reasoning apply to matzo ball soup)?
(This now has a companion question about whether such bread would be practical.)