Distinction: "What can I do you for?" vs. "What can I do for you?"

It's normally a joke.

It's 'funny' because "What can I do you for?" is actually a question that would never be asked, except rhetorically.

Do you, as in "I'm gonna do you in" is what a thug would say before he perpetrated violent acts against you. It could also be used by a police man, for example "Do him for possession", so do him is slang for arrest him.

There is also, the more pertinent definition of do you, which is what a swindler would think when tying to think how to trick you: "What can I do you for?" Where what they mean is "What can I get out of you with my tricks?"

Whereas "What can I do for you?" is someone simply asking how they can help. In the context of a barman, it would be asking what drink or other pub service they can provide.

No one has hit the right tone with this one yet, so I'm going to step in.

What can I do you for?

is, as others have said, a playful inversion of "What can I do for you?" However, it is a bit more than that. It is said as a kind of challenge, announcing that the speaker is identifying himself (again, playfully) as someone who might actually take advantage of the person being addressed. The impression is that the speaker is a sly dog who commonly swindles people and is not likely to be swindled himself. It is playfully confrontational. If said to a friend, it's a joke. If said to a stranger, it can be almost a warning.