I've been noticing more and more Ajax powered pages where at the end
of the normal page end a call is made to fetch more content and is
added to the bottom of the page, in essence making the page longer on
the fly. A good example of this is on http://www.haystack.com
Twitter and Facebook do something similar but a button or link is
needed to expand the content.
I can a usability issue with clicking on a link and then having the
back button not return you to the same 'long' page you left from.