Bookmarklet:

  javascript:var%20b=document.body;var%20GR________bookmarklet_domain='http://www.google.com';if(b&&!document.xmlVersion)%7Bvoid(z=document.createElement('script'));void(z.src='http://www.google.com/reader/ui/subscribe-bookmarklet.js');void(b.appendChild(z));%7Delse%7Blocation='http://www.google.com/reader/view/feed/'+encodeURIComponent(location.href)%7D

The link isn't always there, but it appears at times on the "home" page for Reader.

edit: it will, unfortunately, take you to Reader if it finds a feed, to give you the option of subscribing. So it's not quite Instapaper-like. But, better than nothing :) I get a lot of use out of it.