From the FAQ (page 19 of the 4/4/03 release)
"Tumbling is part of movement, not a separate action. You
can tumble any number of times during a move action, so long
as you do not tumble more than 20 feet or move farther than
your speed. Each time you tumble, you make a separate
Tumble check, using whatever DC is applicable at the time. If
you tumble during a double move, you’re allowed 20 feet of
tumbling as part of each move action.
If you’re using the Spring Attack feat, you’re taking a move
action and the attack action, so you’re moving once and can
tumble a maximum of 20 feet as part of that single move. You
could, however, tumble both before and after the attack."