callback in slideUp does not know the end-state from slide up

callback in slideUp does not know the end-state from slide up

Simplified example;

$('#el').slideUp('fast',callback());

function callback() {
alert($('#el').css('display'));
}

//alerts 'block'. 

I was expecting 'none' as this is the final state after the animation. It seems that this is likely because the application of the 'style:none' attribute is asynchronous with the callback.

is this correct or am I missing something?

Thanks

Justin