SRD:
Total defense [Standard][AoO: No]
Description: A combatant doesn't attack or perform any other activity other than moving at base speed , but the combatant gets a +4 dodge bonus to AC for 1 round. The combatant's AC improves at the start of this action, so it helps against any attacks of opportunity provoked while moving.
"any other activity other than moving base speed". hmmm.
Seems kind of clear if you consider tumbling "an activity",
other than "moving base speed" that is.
Tumble (DEX; TRAINED ONLY; ARMOR CHECK PENALTY)
Retry: An audience, once it has judged a tumbler as uninteresting, is not receptive to repeat performances. The character can try to reduce damage from a fall as an instant reaction once per fall. The character can attempt to tumble as part of movement once per round.
"Can tumble as part of movement" hmmmm.. A little loose
in the wording there.
If your DM rules that tumbling is an "activity" that simply
occurs
during movement, he must rule you cannot
tumble in a round taking total defense. However, if tumbling
is an integrated part of your base move (and not a "special
activity that may occur during a move") then he may rule you
can tumble.
Just reading closely, I come up with tumbling as an activity
you choose to take (or not) during a move and thus,
not available to a character while using the total defense
standard action.
Your DM will be the final judge of course.
~D
For further reference:
Standard Action: A standard action allows a combatant to do something and move a combatant's indicated speed during a combat round. A combatant can move before or after performing the activity of the action.