Bookmark this page in the mobile Safari browser of your iPad or iPhone and then follow this video tutorial to install the Share on Tumblr Bookmarklet.

"Share on Tumblr" Bookmarklet Source Code

javascript:location.href='http://www.tumblr.com/share?v=3&u='+encodeURIComponent(window.location.href)+'&t='+encodeURIComponent(document.title)
iOS Bookmarklets →