On having layout is a mind saving in-depth article discussing internals of Internet Explorer rendering engine, why and how do the hacks work and so on.

I've just fixed a really weird Internet Explorer 7 bug (absolutely positoned element randomly jumping inside relatively positioned one) - it was unstable enough to hide until the finishing stages of the site development. Here's what saved my day:

As a rule of thumb, never position an element relatively without setting layout.

[..]

When possible, try to stick to cases where the containing block has layout and it is the parent of the a.p. [absolutely positioned] element (i.e. there aren't other ancestors in between the a.p. and the containing block).

So its Holly hack to the rescue again. And no, it didn't become obsolete with IE7.