Seeing that several users have showed interest in this (mostly over email, but also Dok), I will post about any information I can find about reflows here.
What Opera has to say about reflows
This is very interesting, because Opera describes that there are two different things that can happen, a reflow, and a repaint. A repaint happens when you change something that doesn't affect the layout of the document, like for example changing the background color of an element. It also talks about how absolutely and fixed elements do not force a reflow upon the whole page, but only the object itself (and that probably due to that even an absolutely positioned element can contain child elements that act according to a flow).
This is very good for us, because it means not only that absolutely positioned elements saves us from most of the pain when it comes to reflows, but also that we can sometimes avoid reflows entirely, and only be bothered by it's baby brother, the repaint.
One prime example for this would be when one has an animation. I'm not quite sure what makes a repaint and what makes a reflow, but, one could think that changing the src of an image tag rapidly to animate it produces a reflow, due to images having diffrent sizes. Perhaps browsers are smart enough to avoid reflowing an image that has a set size, but I doubt it. What, however, we can do, is use the overflow animation trick, which I will not go into detail with here about. I highly suspect that this does not produce a reflow, but a repaint, but I can't be sure (and it's damn hard to test -_-)
The site explains that Opera doesn't do reflows for every DOM change. It tries to wait until the script thread is over (if you don't know what this means, just know that lots of DOM changes after each other will be treated with one single reflow). The exception to this is if the thread takes too long to execute, pushing out the reflow too much. Opera will in this case do the reflow forcefully.
I currently have no information about other browsers than Opera. My gut feeling tells me IE doesn't do this very nicely, but I'm not sure about Firefox. Could go both ways.
I'm working on my testing example, to see if doing these things will help the strain on the computer any (trying to bunch up the DOM manipulations to happen all at once). I shall post here with my results.