How many distinct positive integers can be expressed in the form $ABCD - DCBA$, where $ABCD$ and $DCBA$ are 4-digit positive integers?  (Here $A$, $B$, $C$ and $D$ are digits, possibly equal.)
Clarification:  $A$ and $D$ can't be zero (because otherwise $ABCD$ or $DCBA$ wouldn't be a true 4-digit integer).