I think multiclassing should be an interesting option, but I dislike how munchkin players use it as a cheap way to increase a PC's power. This goes for both taking the 1st level in few core classes and for taking 3-4 PrCls. The problem is that multiclassing penalties are easy to circumvent: they should be either more effective or wiped out completely (there are other RPGs with no classes, just abilities, so why not playing one of them?).
3ed attitude is towards "options, not restriction", and that's very good indeed. 2ed multiclassing rules were too rigid (and dual-classing was awfully nonsense); the 3ed is a HUGE improvement, but heavy-multiclassed PCs tend to become a (useful) bunch of abilities with no real role at all.
When I was a DM, I never used house-rules or extra restrictions on multiclassing, and I was pleased to see my players be singleclass or otherwise have a reason to be more than 1 class. You have a level of Ranger? Fine. Explain why your PC is a ranger to some extent, and Rplay accordingly (if you're partly a Ranger, why do you care nothing at all about nature? how after X levels of Fighter you got a level in Barbarian and your life didn't change a bit? etc.). The ultimate weapon in the hands of a DM is to give bonus Xp for good RPing - although it doesn't seem a common habit - which is usually more difficult for a multiclassed PC (and worse for a multiPrestigeClassed PC).