srjm

Update on that:

function Get_Post_Number($postID){
	$temp_query = $wp_query;
	$postNumberQuery = new WP_Query('orderby=date&posts_per_page=-1');
	$counter = 1;
	$postCount = 0;
	if($postNumberQuery->have_posts()) :
		while ($postNumberQuery->have_posts()) : $postNumberQuery->the_post();
			if ($postID == get_the_ID()){
				$postCount = $counter;
			} else {
				$counter++;
			}
	endwhile; endif;
	wp_reset_query();
	$wp_query = $temp_query;
	return $postCount;
}

will work inside your main loop:

<?php if (have_posts()) : ?>
	<?php while (have_posts()) : the_post();
		$currentID = get_the_ID();
		$currentNumber = Get_Post_Number($currentID);

etc. etc.
Also I’m deliberately ordering them so that the newest post is number 1 but I don’t think it would take much more to order them oldest first if that is what you wanted.

function Get_Post_Number($postID){
	$postNumberQuery = new WP_Query('orderby=date&posts_per_page=-1');
	$counter = 1;
	$postCount = 0;
	while ($postNumberQuery->have_posts()) : $postNumberQuery->the_post();
		if ($postID == get_the_ID()){
			$postCount = $counter;
			echo("true");
		} else {
			$counter++;
		}
	endwhile;
	return $postCount;
}

I was looking for solutions for something similar but there isn’t a lot around in the way of actual answers…
This is just an idea to get started but unfortunately because it’s a second loop you can’t run it inside the main loop without it interfering (it overwrites the_post() global variable I’m guessing). I’ll be looking into running a loop inside a loop soon enough though.