[ColinWright]

That's a weird non-joke version. The ones I know are:

The usual:

    There are 10 kinds of people in the world, those
        that understand binary and those that don't.

Then there was:

    There are 11 kinds of people in the world, those
        that understand binary and those that don't.

Which is a riff on:

    There are 3 kinds of people in the world, those
        who can count, and those who can't.

Then along came:

    There are 10 kinds of people in the world, those
        that understand binary, those that don't, and
        those that didn't expect this joke to be in
        ternary.

Yours, on the other hand, just doesn't really make sense. Maybe I'm missing something ...

[mirkules]

Maybe he's a SQL programmer (a binary column can be true, false or NULL).

[phaemon]

Also:

    There are 11 kinds of people in the world, those
    who understand binary and those who confuse it with Gray code.

[kerbs]

The joke is he doesn't get binary.

...or off by 1 error.

OK, I'm confused too :)