most of my group is multiclassed, actually. I think the rogue is the only one who's not multiclassed; most people have at least one level of fighter.
Of course, I also got rid of mutliclass penalties; me and Angcuru share the same outlook on classes, that they're meant to help you emulate your character the way you see him, not force him into a mold by giving you penalties. I'd only enforce a penalty if I noticed a player getting out of control and trying to powergame it with a big list of multiclasses, and thankfully, I haven't got players like that.