[tibear]

As per the changes to OBR for 2007:

6.05 A batter is out when—
(a) His fair or foul fly ball (other than a foul tip) is legally caught by a
fielder;
Rule 6.05(a) Comment: A fielder may reach into, but not step into, a dugout to
make a catch, and if he holds the ball, the catch shall be allowed. A fielder, in order
to make a catch on a foul ball nearing a dugout or other out-of-play area (such as
the stands), must have one or both feet on or over the playing surface (including
the lip of the dugout) and neither foot on the ground inside the dugout or in any
other out-of-play area. Ball is in play, unless the fielder, after making a legal catch,
falls into a dugout or other out-of-play area, in which case the ball is dead. Status
of runners shall be as described in Rule 7.04(c) Comment.

It is the position of the fielders feet when the ball is caught, not where they were when he jumped. i.e. If the fielder is completely over DBT when the ball is caught it is a foul ball and not an out.