//Presumes your posts code is inside a div "#posts" and ".page" inside this. 

$(document).ready(function() {
   var totalPages = $("#hidden_navinfo .totalpages").text();
   for ( var i = 1, len = totalPages; i < len; i++) {
		var count = i+1;
		var new_posts = "";
		$.get("/page/" + count, function(data) {
		 	$("#posts").append($('.page', $(data)));
		});

	}
});


// This needs to go in the the page div 
<div id="hidden_navinfo"><span class="totalpages">{TotalPages}</span></div>

Very rudimentary script for loading all posts on a tumblr blog at the same time. Needs a lot of work, just a concept really.

Tumblr - load all posts

jQuery