I've had the following code in my personal /monobook.js subpage for ages:
#lastmod { position:absolute; z-index:40; left:155px; float:left; top:2px; }
It moves the "This page was last modified on 10 October 2011, at 21:31" to the top of the window, above the "article / talk / move ..." tabs. It's incredibly helpful, though occasionally it overlaps with the user links ("MZMcBride / my talk / my preferences ...") if my screen is too narrow or too zoomed in.
It should be noted that the timestamp isn't always up-to-date. In rare cases, the HTML cache of the page won't be updated for whatever reason, so the date displayed will simply be wrong. It also doesn't update when templates, images, categories, or links change state, as far as I remember. I don't remember if actions such as page moves or page protections will trigger an update. I'm hesitant to see it put next to the <h1>, but having it more prominent would definitely be good, I think.