“Magic-user/thieves, fighter/clerics, and even the rare but potentially awe-inspiring fighter/magic-user/thief walked the land, like chimeras wrought by strange rites involving Player’s Handbooks, an overactive imagination, and a DNA splicer.
— Mike Mearls on Multiclassing in D&D. via chris. (This refers to the status quo, but read the article to see the approach ahead for D&D4. I’m optimistic.)