Wizards has revealed some details about how multiclassing works in the 4th edition of Dungeon and Dragons.  In a nutshell, the new system works through feats that allow you use abilities from other classes.

...we came up with a system of feats that allow you to borrow abilities and powers from other classes. At 11th level, you can choose to forgo your paragon path in order to further specialize in a second class. This approach lacks the intuitive elegance of the 3E system, but it allows us to tone down or boost a class’s multiclass options as needed. If everything works as planned, you have the flexibility to mix classes without making your character into a juggernaut or a cripple. Combos like fighter/wizard now work much better, while traditional choices like fighter/rogue still function just fine. Going forward, we’ll introduce new feats for new classes, ensuring that all classes play well together.

This system seems to be less of a true multiclass and more like dabbling in another class (to use their words).  Also, you can only multiclass into one other class, as opposed to 3rd edition where there really wasn’t a limit.  I’m very curious how they will handle characters like Elminster that previously had several classes.  In the end, this system seems simpler, but also more limiting.  For better or worse, you can kiss your Cleric 7, Fighter 4, Wizard 8, Rogue 1 combination goodbye.