Water Bob: Power acrobatic tumbling like that is very visually stunning and attractive, and I can certainly see why it makes an impression on people but I'm going to have to disagree with the assement of what is being described in the tumble skill. That is tumbling as a performance art; that is not combat tumbling, and I don't think it would make you particularly hard to hit.
The problem with it is that it isn't reactive, and so against a skilled opponent it doesn't really do you any good at all - or at least no more good than running in a straight line (and at higher speed) would do you. It's going to leave you open to attack, unable to compensate to move away from an attack, and off balance. If anything, the sort of tumbles presented in those videos should draw attacks of oppurtunity and not negate them because my 'common sense rule' for what draws an attack of oppurtunity is 'anything you do that isn't actively defending yourself from attack'.
To me, when tumble is used to negate an attack of oppurtunity, it's quite similar conceptually to useing the ride skill and mounted combat - and the speed and agility of your mount - to negate attacks on your mount. It's not that you take your mount into a somersalt, it's that you simply use your speed to react to an attack in such a way that your skill at riding effectively becomes your AC. I see tumble very much in the same way, as reacting to an attack in an agile way such that the attacker is unable to adjust his attack to your movement.
You seem to have a target, and then you swing and you whiff. Against a skilled combatant, that power tumbling that you admire - while impressive atheletic - presents no obstacle to striking the target because it must be preformed with a regular precision in order for the tumbler to maintain his balance. You know precisely where he will land and can simply balestra to position (forward or backward as needed) and strike.
To a certain extent, I'm sympathetic with you. I don't know how many times I've had players forget that they don't have three (or four) hands, and that they can't wield a sword and a shield, and carry a lantern, while closely inspecting a mcguffin that they've just found. Players are very prone to forgetting that there characters exist in a real place with real physical constraints, and loading up with 80 lbs of bulky gear as if it was of no consequence. If you are worried about that particularly, my suggestion would be rather than punishing the Tumble skill particularly to simply raise the armor penalties from encumbrance and armor across the board.
However, in my experience this is unnecessary if you as the DM don't forget that that real world is not flat and dry and ask for balance, climb, swim, etc. checks at regular intervals as the terrain demands. But by and large, you shouldn't make this part of the game too tedious. You shouldn't make your players describe exactly how they are stowing each bit of gear. Just do a sanity check once a level or so to make sure that they are stowing the gear.