Updated, thanks to Simon Willison:
Say you'd like to pick part of your jQuery call chain dynamically - for example, to removeClass for many elements, but addClass to one special element. Here's how you do it:
var myfunc = some_condition ? 'addClass' : 'removeClass';
$('#someid').html('something')[myfunc]('some_class');
The first line selects the function name as a string. The second line calls the 'html' function (it could be any function; html is just the example in this case), followed by the function with the selected name (addClass or removeClass in this case).
You'd probably use this in the context of a loop, eg:
for (var i=0; i<10; i++) {
var myfunc = some_condition ? 'addClass' : 'removeClass';
$('#something'+ i).html('sample text ' + i)[myfunc]('selected');
}