I just noticed that the April Editorial Calendar shows that a playtest of dual classing rules will be released on Monday and that the full version is scheduled to be released in Players Handbook 3. As an anti-human kind of guy (half-humans are okay), I must admit that I've never used dual classing rules in my gaming career. In fact, I had to go grab my copy of the 1st edition Players Handbook just to figure out how it worked originally. I know, I know, you could dual class in 2nd edition too... but I don't have 2nd edition books anymore. I only kept the 1st edition ones for nostalgia.
For the uninitiated (a category that I technically still belong in), dual classing was a variant of multiclassing that only humans could do. Instead of learning the abilities of several classes at the same time, dual classing allowed a character to essentially "quit" learning the abilities of one class and begin learning those of another.
I'm eager to see how WotC will implement dual classing in 4E. Because I've never used it, I don't have any positive or negative experiences with the original system to form opinions... but I'm a huge fan of multiclassing and a variant will be interesting to see, even if I'm pretty satisfied with the current multiclassing system.