As others have mentioned TWF is no different to the old sword and board...except in style. Any weapon combat that involves two seperate 'weapons' or objects being wielded is more difficult than wielding only one object. For instance, an untrained individual would find it as difficult to be effective with sword and board as with two small-medium sized weapons.
The main difference comes when a combatant looks at concentration on defence or counter attacking. It is easier to defend with a sword(weapon) and shield, as blocking with a shield will often take less finess(yes that is a generalisation). The shield also acts as a psychological barrier to the opponent as a 'wall' they have to get past.
A second blade(weapon) however gives the combatant greater options (mainly due to speed) in counter attacking and in turn forces the opponent to take greater care as to where the next attack will be coming from. Thus psychologically putting them on the back foot or into a more defencive mindset.
Now, the above are what you might consider the polar ends of fighting with an object in each hand. In between you have all the variations of things like sword and axe, blade and buckler, sword and cloak, net and trident, Spear and shield (which I personally feel we don't see enough of) etc. All of which have distinct advantages and disadvantages in either defence or counter.
What really needs to happen is for some mechanic or ability system that can be expanded for all fighting styles. Certainly there should be no negative 'to hit' modification on TWF for a starting character any more than there should be on someone using sword and board. Rather a fighting style should come with distinct advantage/disadvantage in a particular type of combat (which can be improved/overcome with experience).
Bottom line is effectiveness with any combat style should be based on experience...with that style of fighting. The strength of a combat style is always in the wielder or combatants use of it, not the style itself.
Whether this can or will be portraid in 4E I am unable to guess at but would guess it would come in PHBII and only touched upon in the the core PHB. After all, we don't just RPG to kill things and take their stuff now do we.
